Quantifying the promise of 'beyond' Li–ion batteries

Rechargeability over a large number of cycles is required for high overall efficiency. The best Li–air cells today are only rechargeable over a few cycles, and can be operated only at low current rates [26–29]. Rechargeability is affected by multiple concurrent phenomena, such electrolyte decomposition, and the degradation and passivation of the electrodes [30].

Low rechargeability of Li–air batteries is characterized by rapidly increasing overpotentials during cycling, which ultimately reduce the battery's Coulombic efficiency, and passivation of the electrodes by electrolyte decomposition and carbonate formation, which limits the number of cycles due to an increasingly reduced cathode surface area upon cycling. The two effects are closely related to each other because the passivation of the cathode occurs partly due to electrolyte decomposition. Similar electrolyte decomposition at the anode, leading to SEI formation, also contributes to the overpotential increase.

In the nonaqueous Li–air battery, the electrolyte suffers electrochemical and chemical degradation in the environment of Li₂O₂, and forms insoluble fluorides, carbonates, alkyl carbonates, and other species [2]. The highest occupied molecular orbital (HOMO) level was identified as the key descriptor for the oxidative stability of non-aqueous solvents [31], with a deeper HOMO level of the solvent indicating better resistance against oxidative degradation. Additionally, solvated $\text{O}_{2}^{-}$ anions are known to induce proton/hydrogen abstraction from nonaqueous solvents, which are generally weakly acidic, through a nucleophilic attack process. This severely undermines solvent stability in the battery [32]. It has been shown that the propensity of a solvent to resist proton/hydrogen abstraction, via nucleophilic attack or electrochemical adsorption at the surface, is dependent on its acid dissociation constant, pK_a, in its own environment [33]. The higher the pK_a, the higher the resistance of the solvent against proton/hydrogen abstraction. However, it has been found that most solvents obey an anticorrelation between the HOMO level and the pK_a of a solvent [33]. This implies that a solvent with higher pK_a, which implies more stability against proton/hydrogen abstraction, will also have a higher HOMO level, implying more susceptibility towards oxidative degradation. Thus, the ideal solvents desired must be anomalies to this fundamental trade-off between oxidative stability and proton/hydrogen abstraction.

In the early years, carbonate-based solvents were widely investigated with salts like LiPF₆ and these solvents are unstable and form insoluble Li₂CO₃ during discharge instead of the cyclable Li₂O₂, resulting in loss of active components [34, 35]. Ethers and glymes are more stable, have low volatility and low polarity. However, ethers and glymes wet the carbon cathode to reduce the O₂ transport, and also decompose to form Li₂CO₃, HCO₂Li, CO₂, and H₂O [36–38]. Ionic liquids have been proposed as strong candidates for the role of electrolytes, due to their low vapour pressure, high ionic conductivity, wide electrochemical window and stability against $\text{O}_{2}^{-}$ [39–41]. Ionic liquids of the piperidinium and pyrrolidinium families have demonstrated excellent lithium metal cyclability [42]. To avoid problems associated with volatility and electrochemical stability of liquid electrolytes, solid electrolytes based on ceramics and solid polymers have also been explored [43–45]. However, their rechargeability has not been demonstrated.

There is a large number of constraints on the electrolytes needed for the practical Li–air battery. The electrolyte must be stable in the electrochemical environment, must be able to withstand temperature fluctuations, dissolve O₂, have high ionic conductivity, and resist degradation via the various mechanisms described above. It is a formidable task to find an electrolyte which simultaneously satisfies all these requirements. To enable the discovery of such electrolytes, several groups have developed a large database of electrolytes with all the relevant properties. These databases can be easily queried, and will help researchers not only discover better electrolytes, but also discover various trends in electrolyte properties. A broader search for the ideal electrolyte includes electrolyte mixtures, as their combined properties and interaction with each other can produce novel behavior, not possible in pure electrolytes.

Another major challenge with the development of materials for the Li–air battery is to find suitable electrode materials. The highly investigated graphite cathodes are a major source of carbon that form lithium carbonates. These carbonates deposit over the cathode and inhibit charge transport, which causes an increase in the overpotentials, and in turn significantly reduces the efficiency of the battery [30]. Various metal oxides of Mn, Fe, Cu, Pd, Ru, Ir have been explored to search for materials that possess better rechargeability [46–49].

The Li–air battery, as it stands today, cannot be fully discharged to the extent of its theoretical energy density. The primary reason for this limitation is the formation of the insulating, insoluble discharge product Li₂O₂ in form of thin passivating layers over the cathode, thus shutting off the charge transport. Pore blockage by lithium oxides and other parasitic products at the beginning of the discharge limit the O₂ transport at the cathode could also limit the battery capacity.

To produce innovative, functional designs that would prevent electrode passivation and pore blocking, it is important to know the ways in which the cathode microstructure affects discharge performance. Hence, it is necessary to characterize the porous structure of the cathode and relate it to observed electrochemical performance. One solution is to use nanostrucured materials like graphene, nanotubes, foams, and nanofibres, to increase the surface area and lower the charge transfer resistance [50, 51]. However, just increasing the surface area is not sufficient to achieve higher capacity. The cathode structure should also be highly porous, with pore sizes larger than 20 nm to be able to sustain O₂ mass transport [52]. Mesoporous carbon-cathode based Li–air batteries have been shown to have a significantly higher capacity [53, 54].

At higher currents, the capacity is limited by O₂ transport. It is dependent on solubility of O₂ in the electrolyte and its diffusion in the electrode microstructure. O₂ solubility in inorganic solvents is low but it can be enhanced by choosing appropriate salts [55, 56]. The transport of O₂ can also be enhanced by creating gas diffusion channels in the cathode microstructure [57].

Another potential solution to improve battery discharge capacity is to use electrolytes that dissolve the insulating Li₂O₂ film, and re-deposit the compound to form large toroidal deposits by themselves, in the absence of a passivating film [58]. This solution process can be activated by either choosing an electrolyte with a high Gutmann donor number (DN), to stabilize the Li⁺ , or an additive with a high acceptor number (AN), like H₂O, to stabilize $\text{O}_{2}^{-}$. It has already been shown that choosing a high-DN solvent results in increased discharge capacity [59]. However, activating the solution process results in an increase in the formation of $\text{O}_{2}^{-}$, which leads to H-abstraction, as discussed above. Thus, there exists a trade-off between rechargeability and discharge capacity, as has been explained recently [32]. Increasing the DN of the electrolyte also stabilizes dissolved Li⁺ , by forming solvation complexes. The formation of these bulky solvation complexes may decrease the ionic mobility of Li⁺ , which might limit the maximum achievable operating current. Although the solution process leads to an increase in capacity, it is rate-limited by the diffusion of $\text{O}_{2}^{-}$, and hence the increase in capacity will only happen at low current densities [58]. The challenge of improving the discharge capacity at higher currents, without affecting the rechargeability of the battery, still remains an open question.

Sulfur exists as homoatomic rings of various sizes, the orthorhomibic α—S₈ being its most stable allotrope at room temperature [60]. The desired end product of the discharge reaction, Li₂S, forms as a result of a series of reactions which begin with the opening of the cyclo-S₈ ring upon reduction. This leads to the formation of high-order Li–S polysulfides of various moieties [12, 61]. Upon continuation of the discharge process, the higher-order polysulfides are reduced to intermediate and lower-order polysulfides (e.g. Li₂S₂) as given by:

$$\begin{eqnarray}&&{{\text{S}}_{8}}+2{{\text{e}}^{-}}\rightleftharpoons \text{S}_{8}^{2-}\end{eqnarray} \tag{ 7 }$$

$$\begin{eqnarray}&&\text{S}_{8}^{2-}\rightleftharpoons \text{S}_{6}^{2-}+\frac{1}{4}{{\text{S}}_{8}}\end{eqnarray} \tag{ 8 }$$

$$\begin{eqnarray}&&2\text{S}_{6}^{2-}+2{{\text{e}}^{-}}\rightleftharpoons 3\text{S}_{4}^{2-}\end{eqnarray} \tag{ 9 }$$

$$\begin{eqnarray}&&3\text{S}_{4}^{2-}+2{{\text{e}}^{-}}\rightleftharpoons 4\text{S}_{3}^{2-}\end{eqnarray} \tag{ 10 }$$

$$\begin{eqnarray}&&3\text{S}_{4}^{2-}+2{{\text{e}}^{-}}\rightleftharpoons 4\text{S}_{3}^{2-}\end{eqnarray} \tag{ 11 }$$

$$\begin{eqnarray}&&2\text{S}_{3}^{2-}+6\text{L}{{\text{i}}^{+}}+2{{\text{e}}^{-}}\rightleftharpoons 3\text{L}{{\text{i}}_{2}}{{\text{S}}_{2}}\end{eqnarray} \tag{ 12 }$$

$$\begin{eqnarray}&&\text{L}{{\text{i}}_{2}}{{\text{S}}_{2}}+2\text{L}{{\text{i}}^{+}}+2{{\text{e}}^{-}}\rightleftharpoons 2\text{L}{{\text{i}}_{2}}\text{S}\end{eqnarray} \tag{ 13 }$$

One of the fundamental issues plaguing Li–S batteries is the decay in capacity contributed by the solubility of high-order polysulfides Li₂S_x (6  <  x  <= 8), in several organic liquid electrolytes [11, 62]. After their dissolution, these intermediate polysulfides have been found to migrate to and react with the lithium metal anode to form low-order polysulfides, which then migrate back to the cathode and form high-order polysulfides again. This process effectively works as an internal redox 'shuttle', which lowers the Coulombic efficiency of the cell [63], and leads to (i) a severe loss of active material in the cathode and (ii) lithium corrosion in the anode [61].

A critical result of polysulfide dissolution is also the problem of self-discharge. Practical energy storage devices must posses low self-discharge characteristics, which serve as a marker of high Couloumbic efficiency. Li–S batteries, however, are known to demonstrate a high degree of self-discharge, like conventional batteries. This is due to the continuous and slow dissolution of higher order polysulfides in nonaqueous electrolytes in the system, even in the resting state, because of the presence of gradients in the chemical potential of the active material. This process leads to severe loss of the active material from the cathode, and eventually results in a lowered open-circuit voltage and decreased discharge capacity. Moreover, the diffusion of the dissolved material outside of the cathode region renders it permanently unusable. The amount of self-discharge has been found to depend on the type of current collectors employed in the system [64], necessitating the search for better current collectors at the electrodes.

In recent years, researchers have attempted to improve the utilization and cycling life of active materials by adding additives into the electrolytes. Using LiNO₃ as an additive, it has been shown to improve the performance of Li–S batteries [65]. LiNO₃ was found to react with the electrolyte, and lithium polysulfides have been found to form a protective passivation layer on the surface of the lithium anode. Another additive, P₂S₅, was found to promote dissolution of polysulfides, and at the same time, passivate the lithium metal surface. This yielded a structure that was conductive for lithium ions, but prevented the access of polysulfides to metallic lithium [66]. Adding Li–S polysulfides themselves to the organic electrolyte has been found to limit their migration from the cathode, due to the common-ion effect [12].

Another popular approach to suppress the shuttle mechanism is the encapsulation of the polysulfides within the cathode structure. Encapsulation of sulfur into hierarchically porous carbon nanoplates, and microporous carbon polyhedrons derived from a metal-organic framework, has been reported to exhibit improved specific capacities and rechargeability [67–69]. Additionally, polymer materials such as sulphur-polypyrrole, -polyaniline, and -polyacrylonitrile composites, have been developed, with the target of reducing dissolution and the shuttle effect of polysulfides, as well as improving electronic conductivity [70–72].

The insulating nature of both sulphur and Li₂S necessitates the inclusion of conductive additives and binder materials into the electrodes, so that the derived structures can shorten the transport pathway for both ions and electrons during prolonged cycling. Sulphur encapsulated into hierarchically porous carbon has been shown to have good conductivity, with porous channels effectively adsorbing the active materials through physical adsorption. This leads to improved performance of Li–S batteries [67, 73, 74]. Conducting polymers are also known to form facilitating coating layers, which demonstrate optimum electrochemical performance [72, 75].

The conversion reaction during electrochemical discharge in the Li–S battery is accompanied by unfavourable morphological changes in the cathode that lead to volumetric changes of approximately 80% [8]. Due to the stresses induced by such large volume changes, this leads to an unstable electrode morphology, which is extremely detrimental for long term cycling. Therefore, flexible and robust conductive matrices are indispensable to accommodate the Li–S discharge products. Several modifications to the cathode material have been proposed to absorb the strain from cycling-induced volume changes. Graphene and graphene-oxide based materials with high surface area, superior electronic conductivity, high mechanical strength, and flexibility, have been found to be suitable to confine the polysulfides, accommodate the volume changes during discharge, and provide stable cycling [7, 76]. Apart from these, sulphur–metal–oxide core–shell and yolk–shell materials have also been devised, to provide extra void space to cushion the volume expansion of the active material. Such a solution would allow the battery to retain its structural integrity, and also assist with the retention of polysulfides [77, 78], although sulphur loading in such materials needs to be increased for practical applications.

The primary functions of electrolytes in the Li–S battery are to efficiently transport lithium ions between the electrodes and protect the metallic lithium anode, through the formation of a lithium ion conductive solid electrolyte interphase (SEI). At the same time, it is required that the electrolyte is physically and chemically stable under the operating temprature and pressure, and also electrochemically stable within the working voltage window. The electrolyte has a critical effect on the discharge reaction mechanisms and the solubility of the formed products. Several electrolyte schemes have been applied to Li–S batteries, such as ether- and carbonate-based organic electrolytes [62, 79], ionic-liquid-based electrolytes [80], and polymeric and non-polymeric solid electrolytes [61, 81].

While an immense amount of effort has been invested in the design of new cathodes, in order to counter the insulating nature of sulphur and accommodate massive volumetric changes, the other longstanding problem, the dissolution of polysulfides, can be addressed by electrolyte engineering. A particular advantage of liquid electrolytes is that an optimum balance of desired properties can be achieved by taking various blends of solvents, salts and electrolyte additives, and combining them in a synergistic manner. Chain-ether-based liquid solvents, such as dimethoxyethane (DME), polyethylene glycol dimethyl ethers (PEGDME), and tetraethylene glycol dimethyl ether (TEGDME), have been the most commonly used solvents, due to their inhibition of polysulfide dissolution, high ion conductivity, low viscosity, and good wettability [82, 83]. At the same time, lithium salts in the electrolyte require expeditious solubility, a high degree of dissociation, and good compatibility with the liquid electrolyte. Salts such as LiTFSI and LiTf have been the most popular, due to their thermal stability and high dissociability [81]. The emergence of solid electrolytes has shown promise as the next breakthrough in electrolyte systems, largely by eliminating the polysulfide shuttle mechanism, and making the battery intrinsically safer; however, a number of challenges regarding ionic conductivity, efficiency of the discharge mechanism, and interfacial resistance, need to be solved [61, 81].

The most severe drawback faced by current Na–air battery is that they generally exhibit poor capacity retention upon cycling [17, 18, 84]. This issue is believed to be primarily caused by the properties of the battery's discharge product, sodium superoxide (NaO₂) [17]. Typical rechargeable batteries, such as the Li–air battery, form a thin film of discharge product on the cathode, just a few nanometers in thickness. Such a film covers much of the cathode's available area, and has few large features, unless operated at exceedingly low currents, at which point large, 3-dimensional features of the order of microns start to appear [58]. In the Na–air battery, on the other hand, the discharge product builds up in the form of very large localized cubes of sodium superoxide, which have been observed to reach sizes as large as 50 microns on a side, with no film-like discharge product observed at all. This is markedly different behavior from that of the Li–air battery, both in the pattern of deposition and the stoichiometry of deposition: Li₂O₂ versus NaO₂. Preliminary suggestions to explain this difference relate the energies of formation of nanoscale Na₂O₂ and NaO₂, but a significantly more in-depth study of this is necessary [85].

The mechanism of the growth process of these large crystals is not fully understood. Recently, it has been suggested that the major mechanism responsible for both the growth and dissolution of the NaO₂ crystals is phase-transfer catalysis, akin to the solution process, where in $\text{O}_{2}^{-}$ anions are transported being bound temporarily in a HO₂ unit, where the H⁺ proton is donated by the electrolyte [86]. It has been observed that upon cycling, the cubic crystals of NaO₂ do not decompose completely, but prefer to dissolve only at the corners and edges and the charging overpotential increases steadily.

Another major issue concerning Na–air battery development is closely related to the first issue of rechargeability. The decomposition of the electrolyte and salt start to occur at high recharging voltages, which are required to decompose the discharge product. This decomposition leads to the formation of passivating compounds which are harmful to the battery, including various carbonates and complex molecules containing fluorine and sulphur when sodium triflate (NaSO₃CF₃) was used as the salt [17]. These compounds formed due to parasitic reactions have been observed to coat the remaining discharge product crystals as thick, wispy, impenetrable layers, with measured thicknesses on the order of microns, which decrease the battery's ability to successfully be cycled by shielding the remaining NaO₂ from external electrochemical reactions. This shielding effectively passivates the entire cathode, cutting off both the open carbon cathode, as well as the remaining undissolved NaO₂ cubes [17]. These parasitic processes lower the Coulombic efficiency of the battery.

There exists a similarity between the insulating deposits in the Na–air battery and the carbonate formation in the Li–air battery. Both are mainly caused by electrolyte decomposition, and both eventually form impenetrable and insulating deposits that inhibits subsequent battery cycling. Whereas in the Li–air battery the carbonates form largely uniform and featureless layers, the insulating deposits on the Na–air discharge product crystals have a pronounced, disordered, wispy structure. It has also been found in measurements that while the Na–air battery produces negligible amounts of carbonates during the charge cycle, it produces a larger amount of carbonates during the discharge cycle, when compared with the Li–air battery [87]. The exact mechanism of formation of these complex deposits is not well understood, and is an important issue that needs to be solved, if the Na–air battery is to be developed into a practicable system.

A critical part of battery development is the laboratory experimental stage, as it is during this phase that the fundamental electrochemical processes and mechanisms are identified and characterized. Experiments typically study the discharge behavior of the battery, the effects of doping the anode and cathode with additional materials, the effects of certain additives on the electrolyte that improve battery reliability and capacity. Proper, thorough experimentation paints a complete picture of battery dynamics, and allows to choose optimal materials for most efficient performance.

One of the main types of battery cells used in laboratory research for BLI chemistries is the Swagelok cell [91]. When used to study the Li–air battery, the Swagelok cell is constructed from a lithium metal foil anode and a porous carbon cathode, in between which lies a separator soaked in electrolyte. The cell is provided pure O₂ gas, flowing to the cathode, to avoid contamination and parasitic reactions from other atmospheric gases. An additional feature used in the study of oxygen-breathing batteries is a mass spectrometer, to study the consumption and production of oxygen during the discharge-charge cycle, using a method called differential electrochemical mass spectroscopy [91]. This addition allows researchers to relate the chemical measurements (oxygen consumption and production) to the electrical measurements (voltage and current) of the battery, and provides a deeper understanding of the processes taking place within the battery.

Another type of cell used in laboratory investigation of batteries is the bulk electrolysis cell, used to characterize the fundamental electrochemistry associated with the battery [91]. Such a cell differs from the Swagelok cell primarily by the addition of a third electrode, used for reference. Also, the porous carbon in the cathode is replaced by a layer of glassy carbon, and the electrolyte-soaked separator is replaced by a reservoir of liquid electrolyte, with the gaseous oxygen pumped directly into the liquid. As the cell is cycled, the liquid is stirred, to speed up transport of lithium ions and oxygen [91]. The bulk electrolysis cell has been used to successfully understand the fundamental electrochemistry of Li–air batteries [91].

The manufacturing processes available for the production of battery components can be separated into high-temperature and low-temperature processes. High-temperature processes are undesirable because of the associated energy costs and emissions. More than 400 kWh of energy can be required to produce a single 1 kWh Li–ion battery, a process that emits 75 kg of CO₂ [92]. Thus, low-temperature production methods are important to develop. Three metrics have been identified in efficient process development: economy of atoms, economy of energy, and control of size, shape, and texture. One efficient process for producing electrode materials that performs well on these metrics is Ionothermal Synthesis, which has enabled researchers to manufacture phosphates and silicates around 200 °C, rather than the 700 °C required for standard high-temperature ceramic processes [92, 93]. Even lower manufacturing temperatures can be attained by using biological processes. These can be used for a surprising variety of products: the Bacillus pasteurii bacterium has been used for biomineralization of calcite, and the tobacco mosaic virus has been used as a template for nanostructured V₂O₅ cathodes [92]. If the right species of bacteria can be identified, or grown, mass-production of Li-based electrodes could be carried out at room temperature.

In recent years, with the rise of affordable and widespread additive manufacturing equipment, experiments have begun with less conventional manufacturing techniques for batteries, such as 3D Printing and ink-printing of battery components, or even entire batteries [94, 95]. Such experimental techniques allow to create battery geometries that are impossible to replicate with traditional manufacturing methods, and may prove to be a practical solution to yet unsolved difficulties in the development of robust metal–air batteries. Another deviation from standard battery design is the introduction of ionic liquids as the battery reaction media [92]. These are cheaper to produce than complex anions, such as TFSI, can be easily reused or recycled, and use biomass precursors, which can be easily created in bulk. The need for electrolytes stable against parasitic processes such as nucleophilic attack by $\text{O}_{2}^{-}$ could be solved by using ceramic membranes in two-compartment cells, which would improve overall electrochemical stability [92]. Manufacturing methods being explored today avoid many of the limitations of earlier manufacturing paradigms, which suffered from high energy consumption and low thermal efficiency.

To be practical for worldwide application, the production of new batteries will have to be scaled up to industrial output quantities. This will require the development of a proper and efficient mass-production system and process flow. An obvious starting point for battery manufacturers in the near future is to modify their current manufacturing processes to fit the requirements of the new batteries. The battery industry, as well as the fuel cell industry, is rich with proven and perspective manufacturing techniques, many of which can be adapted, modified, or combined to answer the production needs of novel battery designs. This is likely to save a significant investment of time and funding, especially as many stages of the manufacturing process do not involve the fundamentals of battery performance, but rather superficial details like packaging. An example of a suggested process flow, modified for beyond-lithium–ion battery assembly is discussed below.

Process flow

The specific process used for battery production plays a significant role in determining the resultant cost of battery manufacturing, as shown in figure 8. The production process begins with the acquisition of raw materials. Materials necessary for the production of the positive and negative electrodes, and the electrolyte, make up the highest raw material purchase costs. In this analysis, the solvent to be used for the battery electrolyte is chosen as 1,2-dimethoxyethane (DME). The price of DME is $134 l⁻¹, which is cheaper than ethylene carbonate but costlier than propylene carbonate. Li and TiC are chosen as the anode and cathode materials for the process flow diagram. The TiC-based cathode exhibits an energy storage capacity comparable to the graphite cathode, but suffers from significantly lesser parasitic reactions. It also serves as a more reliable base for the formation-decomposition of Li₂O₂ than pure graphite. After initial electrode preparation, the coated electrode foils are cut into strips and dried by heating under vacuum. This is one of the critical stages of battery manufacturing as moisture must be eliminated during this process to prolong battery life. The control laboratory is then used to verify the quality of raw materials and the produced electrodes.

The next phase, assembly, involves stacking the cells, welding the current collectors and placing the cell inside an enclosed container to be filled with electrolyte and sealed. The sealed container is placed into a dry-room atmosphere, kept at a maximum dew-point temperature of  −40 °C. Sealed cells are monitored in this environment as they are cycled and tested for charge retention through a full charge-discharge-recharge cycle. Defective cells are rejected and recycled, and the accepted cells are passed on to the AHU, where they are exposed to pure oxygen. The oxygen in this AHU is purified of contaminants by means of an Air Products Dehydration Membrane composed of hollow polymeric fibers, chosen here as the Prism Cactus membrane (model PC4030-D2-6A-20). The unit price of this membrane is between $4000 and $7000. Once the cells are processed with purified oxygen, they are finally assembled into battery packs. The battery packs are discharged to verify overall impedance, and then recharged to a stable level for long-term storage and shipping.

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The battery management system (BMS) is critical for controlling the charging and discharging of the battery and maintaining its overall safety and reliability. The BMS also handles cell balancing and thermal management of the battery pack, which is necessary to avoid reduction in performance and ensure safety against explosion of batteries due to overcharging or abuse, and is expected to predict the behavior of the battery over its lifetime [96–98]. The BMS consists of sensors, actuators and controllers which monitor and control the battery state [96]. BMS has a distributed architecture comprising of the battery control unit (BCU) and the battery measurement unit (BMU). The BCU is used to compute the battery parameters and send control order to BMU. The BMU is responsible for measuring cell votages, total voltage, current, insulation resistance and temperature [99, 100]. Power and energy prediction is required to calculate the available energy for functioning at the given point of time and maximum power that is instantaneously available if required at that time. For power and energy prediction, a good BMS should also be able to predict the degradation of battery components over time. Power and energy prediction is done by monitoring the bulk state of charge (SOC) and surface SOC [101]. The bulk SOC is calculated by integrating the measured current over time. Safety and reliability is monitored by measuring and controlling the voltage and temperature of the battery and a quantity called state of health (SOH). The SOH is a figure of merit of present condition of battery compared to its ideal conditions. The SOH quantifies the degradation of the battery over time [98, 102]. In addition to this, the BMS also calculates various model parameters like film resistance, diffusion coefficients, etc, to maintain the accuracy of the model used for prediction.

For controlling the battery efficiently, a good model for the battery is required. Various models have been used to predict the battery behavior. The most typical BMS employ equivalent circuit models, which are based on representing the battery in term of an electric circuit based on resistors and capacitors whose resistance and capacitance is fitted using experiments [103–105]. Alternatively, physics based electrochemical models have also been developed to overcome the limited prediction capability of equivalent circuit models. Physics based electrochemical models are based on the electrochemical equations describing the battery dynamics and hence are far more accurate in spite of the assumptions and approximations made [97]. The electrochemical models are one dimensional and assume the electrodes to be comprised of spherical particles whose size varies with material deposition/removal during charging/discharging and a separator between the electrodes to describe the diffusion of cations across the electrodes [106–108]. The equivalent circuit models are computationally inexpensive but have low accuracy over a wide range of electrochemical potentials and don't capture degradation effectively. The physics based models are quite accurate, but computationally quite expensive as they involve solving a set of non-linear PDEs. The complexity of the controller also depends on the complexity of the model used. Thus a computationally cheap model that captures the physics of the battery has to be developed.

One major challenge for BMS is accurate and synchronous measurement of the battery current and the voltage of the pack cells, along with data communication over a number of voltage domains and fulfillment of ASIL-C safety requirements. Voltage accuracy of 1–2 mV at the cell level and 0.1% at the pack level, current measurement with an accuracy of 0.5–1% upto 450 A and temperature accuracy of 1°C for  −10 to 100 °C is required [99, 100, 109]. SOC, SOH and other battery parameters need to be predicted with an accuracy of 5%. Other challenges for the development of BMS include building an analytically tractable model for capturing the physics of the batteries, building efficient methods for determining and calculating battery state variables as bulk SOC, surface SOC and SOH in real time and designing quick offline estimation strategies for identifying the battery model parameters. A few physics-based models have been developed for Li–air batteries and however, these are still in the nascent stage and require further validation [110–112].