Development of a benchmarking model for lithium battery electrodes

Highlights

• A top-down benchmarking model for lithium battery electrodes is developed.

• For HEVs, state of the art electrodes (graphite, LTO and LFP) are most suitable.

• TiO₂ and LFP are most interesting for stationary batteries due to cost and lifetime.

• New chemistries have to be optimized for cells in BEVs due to the energy density.

• Li₂MnO₃·LiNi_0.5Co_0.5O₂ or LiNi_0.5Mn_1.5O₄ with silicon anodes can reach 330 Wh/kg.

Abstract

This paper presents a benchmarking model to enable systematic selection of anode and cathode materials for lithium batteries in stationary applications, hybrid and battery electric vehicles. The model incorporates parameters for energy density, power density, safety, lifetime, costs and raw materials. Combinations of carbon anodes, Li₄Ti₅O₁₂ or TiO₂ with LiFePO₄ cathodes comprise interesting combinations for application in hybrid power trains. Higher cost and raw material prioritization of stationary applications hinders the breakthrough of Li₄Ti₅O₁₂, while a combination of TiO₂ and LiFePO₄ is suggested. The favored combinations resemble state-of-the-art materials, whereas novel cell chemistries must be optimized for cells in battery electric vehicles. In contrast to actual research efforts, sulfur as a cathode material is excluded due to its low volumetric energy density and its known lifetime and safety issues. Lithium as anode materials is discarded due to safety issues linked to electrode melting and dendrite formation. A high capacity composite Li₂MnO₃·LiNi_0.5Co_0.5O₂ and high voltage spinel LiNi_0.5Mn_1.5O₄ cathode with silicon as anode material promise high energy densities with sufficient lifetime and safety properties if electrochemical and thermal stabilization of the electrolyte/electrode interfaces and bulk materials is achieved. The model allows a systematic top-down orientation of research on lithium batteries.

Introduction

Lithium Ion Batteries (LIBs) are of special interest with respect to fuel cell battery hybrid systems for light traction (0.5–5 kW) and individual transport (5–200 kW), as well as for battery electric vehicles (BEVs) and stationary applications [1], [2], [3]. The batteries in these systems must fulfill multiple requirements. An increased volumetric E_V and gravimetric energy density E_m is essential to enabling the achievement of higher operational ranges by BEVs and reducing the mass and volume of fuel cell power trains. The power density P_m of the battery determines the power capability of BEVs and the possible peak load of hybrid fuel cell systems. Increased cyclic and calendaric lifetime, improved safety characteristics, the usage of more abundant raw materials and low specific costs are further important factors for all considered applications. Conventional LIBs must be modified to reach these goals. For this reason, the introduction of novel cell chemistries is required. This paper outlines a benchmarking model that draws on the fundamental material characteristics of lithium battery electrode materials so as to systematically and quantitatively identify their potential for application in different battery and fuel cell hybrid power trains.

Table 1 lists the requirements for the battery in a hybrid electric vehicle (HEV), a BEV and a stationary energy system, as communicated by the Japanese New Energy and Industrial Technology Development Organization (NEDO) [4] and the US Department of Energy (DOE) [5] for the years 2020 and 2030. Additionally, the derived target values for the battery of an in-house, 1 kW direct-methanol fuel cell (DMFC) hybrid system for light traction electric vehicles (LHEVs) are listed [6]. The characteristics of a commercial LIB comprised of a carbon-based anode, a LiNi_0.8Co_0.15Al_0.05O₂ cathode and an organic solvent-based electrolyte from Gaia are opposed [6].

Based on the discrepancy of the target values and LIB-characteristics, state-of-the-art systems must be optimized with different prioritizations. Table 1 specifies the respective priorities from 0 for a low priority to ++ for a high priority. BEV batteries place a high demand on volumetric and gravimetric energy density and costs. The power densities of present systems fulfill the demand of BEVs, while the cyclic and calendaric lifetimes must be increased.

In contrast, HEV and LHEV batteries have high priorities in terms of power density and lifetime. The gravimetric energy density of the exemplary LIB has to be increased by 20% and 36% for HEV and LHEV batteries, respectively. The higher value required for LHEVs arises from the lower limits for battery mass and volume in the comparably smaller systems. Due to the high volumetric energy density of conventional batteries, they attain the requirements of the hybrids considered. State-of-the-art LIBs reach the desired energy and power densities needed for stationary applications, while their calendaric and cyclic lifetime, as well as their costs, must be significantly optimized. According to the derived prioritization for the additional parameters of raw materials and safety, all applications must fulfill high safety standards, whereas the utilized resources are more important for large-scale BEV and stationary batteries.