ReviewRecent developments in cathode materials for lithium ion batteries
Introduction
The development of improved battery technology is critical for advancements in a variety of applications ranging from hybrid electric vehicles to consumer electronics [1], [2], and improved battery performance depends on the development of materials for the various battery components [3], [4], [5], [6]. Most lithium ion batteries use organic solvents as the electrolyte, the most common being LiFP6, which has a low electrical resistance [7], and is typically mixed with carbonates. Solid electrolytes, including polymers [8] and inorganic compounds [9], [10], are used for solid state batteries, which have advantages in terms of miniaturization and durability. The most common anode materials are carbon-based compounds and lithium-containing alloys. Both approaches result in the establishment of a reduced lithium activity (as compared to lithium metal), which reduces reactivity with the electrolyte and improves safety, but also leads to a lower cell voltage. There are efforts in the development of improved electrolyte and anode materials, but the focus of this paper is on the cathode materials.
Cathode materials are typically oxides of transition metals, which can undergo oxidation to higher valences when lithium is removed [11], [12]. While oxidation of the transition metal can maintain charge neutrality in the compound, large compositional changes often lead to phase changes, so crystal structures that are stable over wide ranges of composition must be used. This structural stability is a particular challenge during charging when most (ideally all) of the lithium is removed from the cathode. During discharge lithium is inserted into the cathode material and electrons from the anode reduce the transition metal ions in the cathode to a lower valence. The rates of these two processes, as well as access of the lithium ions in the electrolyte to the electrode surface, control the maximum discharge current. Exchange of lithium ions with the electrolyte occurs at the electrode–electrolyte interface, so cathode performance depends critically on the electrode microstructure and morphology, as well as the inherent electrochemical properties of the cathode material. For example, there is considerable work on the use of nanostructured electrodes with high surface and interfacial areas to improve performance [13], [14], [15], [16], [17]. While this paper will include some discussion of general microstructural features, the focus is on the cathode materials rather than the microstructures.
Section snippets
Cathode materials
The cathode material most commonly used in lithium ion batteries is LiCoO2 [18]. LiCoO2 forms the α-NaFeO2 structure, which is a distorted rock-salt structure where the cations order in alternating (1 1 1) planes. This ordering results in a trigonal structure and, for LiCoO2, planes of lithium ions through which lithiation and delithiation can occur [19]. Although LiCoO2 is a successful cathode material, alternatives are being developed to lower cost and improve stability. Cobalt is less
Cathode performance
The multitude of materials, geometries and operational variables in lithium ion batteries complicates comparison of the performances of different cathode materials. Although there are a few reports in which different types of electrodes are tested in the same conditions and compared on a single plot (e.g. [47]) most reports focus on a particular type of electrode material with variations in composition or microstructure. On the other hand, summaries of results from different sources (e.g. [151]
Composite cathodes
The combination of two electrode materials to form a composite electrode can be used to improve performance [211]. For example, the addition of LiFePO4 to other electrodes, including LiCoO2 [178], [212], Li(Li0.17Mn0.58Ni0.25)O2 [212] and Li(Ni0.5Mn0.3Co0.2)O2 [213], improves capacity retention during cycling and performance at high discharge currents. Similarly, a phosphate surface treatment can improve capacity [214] or performance after cycling [215] of oxide electrodes. Monoclinic (C2/m) Li2
Effect of doping
The performance of cathode materials can be improved by doping, but the interpretation of doping effects can be complicated by the interrelations between doping and microstructure and morphology, since the microstructure formed can be affected by the dopant additions. Some examples in which the effects of doping on the electrochemical properties of the electrode are attributed to the effects of the dopant on the cathode microstructure or morphology rather than the effects on the material
Effect of microstructure and morphology
As mentioned above, electrode performance depends on the electrode microstructure and morphology. Although the focus of this paper is on the materials rather than morphology, some general aspects of electrode morphology will be discussed. Intercalation and deintercalation occur along specific crystallographic planes and directions, so higher crystallinity improves electrode performance (e.g. LiCoO2 [387], LiMn2O4 [388], Li1.02Mn1.5Ni0.5O4 [389], LiFePO4 [155], [390], [391], [392].
The electrode
Conclusions
The development of improved cathode materials is a challenge for meeting current and future energy storage requirements. Several transition metal based cathode materials can provide high voltages and good capacities. Full utilization of these materials for numerous recharging cycles and at high discharge currents continues to be a challenge. Specifically, stabilizing the desired crystal structure, especially during delithiation, and preventing reaction with the electrolyte are important for
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