High Voltage Lithium Ion Batteries
Today, Li-ion batteries (LIBs) face the challenge of application in electrified vehicles (EVs), which require increased energy density, improved abuse tolerance, prolonged life, and low cost. LIB technology can significantly advance through stabilizing the high-voltage and high-energy cathodes based on the Ni-rich [LiNixCoyMnzO2 (NCM) (x→ 1)], Li, Mn-rich (LMR-NCM, Li1+xNiyCoyMnzO2, 0.1 < x < 0.2, z > 0.5), LiNi0.5Mn1.5O4 (LNMO) spinel cathodes. While the cycling stability of such cathode materials during cell operation tends to decrease for several reasons. Our group has been working for many years to develop different strategies to stabilize the electrochemical performance.
Li, Mn-rich (LMR-NCM) cathode materials
The high-capacity Li, Mn-rich NCM cathode materials have been considered as the potential candidates for the upcoming battery era because of their high reversible capacities above 250 mAh/g, which can lead to the high energy density of ≈800 Wh kg−1 (based on electrodes’ materials, using graphite as the anode material) and cost-effectiveness since manganese is an abundant element and its compounds are relatively cheap. However, this class of cathode materials is a great deal as they suffer from low-rate capability, capacity fading, and severe discharge voltage fall during prolonged cycling. Amongst notable drawbacks like surface reactions, oxygen release, large pore formation, and dislocations, voltage fading, high voltage hysteresis, which occurs probably due to cation migration and layered-spinel transition, are crucial topics to be resolved. The structural dislocations, evolution in Mn and Co redox couples, and change in oxygen-layer stacking (at surface-near region) in the lattice are directly allied to the voltage fading. Continuous structural transformations, oxygen release, and lattice rearrangements during prolonged cycling are the most probable phenomena responsible for declining electrochemical performance. For improving the electrochemical performance of HE-NCM cathode materials in LIB, numerous modifications were proposed, such as lattice oping, surface coating, and reactive gas treatments, as shown schematically in the figure below.
Ni-rich (NCM) cathode materials
Besides the LMR-NCM cathodes, another option for increasing the energy contents of LIBs is the use of Ni-rich NCM. The higher the Ni content in NCM, the higher its specific capacity, up to 240 mAh/g limit posed by LiNiO2 (LNO). The clear advantage of Ni-rich NCM is a high capacity can be extracted with charging potentials below 4.3 V versus Li/Li+. LIB with such cathodes can thus be fully charged without reaching potentials that impair the stability of LiPF6/carbonates electrolyte solutions. However, due to their higher Ni content, these materials are more sensitive mechanically (fatigue cracking upon prolonged cycling) and electrochemically (more reactive in side reactions with the electrolyte solutions). Figure a shows the phase diagrams of three individual lithiated oxides LiNiO2, LiCoO2, and LiMnO2 with various Ni, Co, and Mn compositions. Although Ni-rich NCM cathode materials offer high capacity, as shown in the green area, further improvements and optimization are needed to realize their full potential as cathode materials for LIBs. In addition, four problems are exacerbated with the increase in Ni content: i) cation mixing between Li and transition metals during synthesis, mainly Li+/Ni2+, which interferes with the mobility of Li ions; ii) formation of cracks during prolonged cycling (Figure b); iii) surface reactivity; and iv) low thermal stability (Figure c). Doping and coating are the most common and effective strategies to stabilize this class of materials.
LiNi0.5Mn1.5O4 (LNMO) spinel cathodes
The commercialization of Co-free high-voltage cathode materials, like, LiNi0.5Mn1.5O4 (LNMO), which holds several outstanding properties like 3D channels for high-rate Li+ ions diffusion, high operating potential (~4.7-4.8 V) with a moderately high energy density (up to 650 Wh kg-1), better thermal stability and relatively low cost, is comparatively behind and underrated as well. We believe all of these outstanding inherent properties of the high-voltage spinel LNMO can make it an economical and promising cathode candidate for commercial LIB technologies. However, our primary object is addressing the two challenging issues of (1) limited cycle life and (2) fast capacity degradation of LNMO cathode. Cationic and anionic doping and surface coating are the common approaches for stabilizing the LNMO’s electrochemical performance.
Related articles
1. Nayak, P. K., Erickson, E. M., Schipper, F., Penki, T. R., Munichandraiah, N., Adelhelm, P., ... & Aurbach, D. (2018). Review on challenges and recent advances in the electrochemical performance of high capacity Li‐and Mn‐rich cathode materials for Li‐ion batteries. Advanced Energy Materials, 8(8), 1702397.
2. Martha, S. K., Markevich, E., Burgel, V., Salitra, G., Zinigrad, E., Markovsky, B., ... & Saliyski, N. (2009). A short review on surface chemical aspects of Li batteries: A key for a good performance. Journal of Power Sources, 189(1), 288-296.
3. Erickson, E. M., Sclar, H., Schipper, F., Liu, J., Tian, R., Ghanty, C., ... & Aurbach, D. (2017). High‐Temperature Treatment of Li‐Rich Cathode Materials with Ammonia: Improved Capacity and Mean Voltage Stability during Cycling. Advanced Energy Materials, 7(18), 1700708.