Challenges and Progress in Li and Mn-Rich High Capacity Cathodes for Li-Ion Batteries
Authors: Aurbach Doron , Nayak Prasant , Grinblat Judith , Levi Mikhael D. , Levi Elena , Markovsky Boris 
Published: February 2020

Rechargeable Li-ion batteries have emerged as major power sources for portable electronic devices due to their high energy density and long cycle-life. Layered lithiated metal oxides such as LiCoO2, LiNi1/2Mn1/2O2 and LiNi1/3Mn1/3Co1/3O2 etc. are well studied cathodes which exhibit specific capacities ≤ 160 mAh g-1 with an upper potential limit of 4.3 V vs. Li [1-3]. Higher capacities can be extracted from layered metal oxide cathodes by cycling to upper potentials of about 4.5 V. However, these layered cathodes suffer from capacity fading when cycled to potentials above 4.3 V due to structural instability [4,5]. In recent years Li and Mn-rich cathode materials can exhibit specific capacities ≥ 250 mAh g-1in a wide potentials range of 2.0-4.8 V. However, these integrated layered cathodes suffer from capacity fading as well as average discharge voltage decay due to structural layered-to-spinel transformation and migration of transition metal cations to Li sites upon prolonged cycling [6]. Doping these materials with some cations was found to suppress the phase transition thus stabilizing the structure and hence improving the electrochemical performance [7]. For instance, we found that doping of cations such as Al is able to suppress the voltage decay and stabilizing the capacity upon prolonged cycling of these Li-rich cathodes by mitigating the migration of cations as well as suppressing the layered-to-spinel transformation [8].

Another cathode material that can promise high energy density is the spinel LiNi0.5Mn1.5O4, which can exhibit a specific capacity >130 mAh g-1 at relatively high constant potential, around 4.7V, when being cycled in the 3.5-5V potential domain. Higher capacities ≥ 200 mAh g-1 can be obtained from these cathodes by cycling them in a wider potential range of 2.0-5V.  However, their cycling at a wide potential domain is accompanied by a pronounced fading of the high capacity thus obtained, due to structural instability arising from Jahn-Teller distortion [9,10]. It is possible to develop Li and Mn-rich high capacity integrated spinel-layered cathode materials, comprising both spinel and monoclinic Li2MnO3 phases. For instance, Lee et al. reported a specific capacity of about 200 mAh g-1 for xLi[Li0.2Mn0.6Ni0.17Co0.03]O2.(1-x)Li[Mn1.5Ni0.425Co0.075]O4 (x=0.5 and 0.75) in a wide potential range of 2.0-5.0 V [11]. In 2015, Bhaskar et al. reported high specific capacities ≥ 200 mAh g-1 for x{0.6Li2MnO3·0.4[LiCo0.333 Mn0.333Ni0.333]O2}·(1–x) Li[Ni0.5Mn1.5]O4 (x=0, 0.3, 0.5, 0.7, 1) and found x=0.5 as the optimum for the high energy and high power performance of layered-spinel composite cathodes [12]. We also synthesized layered-spinel cathodes such as Li1.17Ni0.25Mn1.08O3, LiNi1/3Mn2/3O2 and LiNi0.33Mn0.54Co0.13O2by self-combustion based synthesis and demonstrated their better electrochemical performance as compared to cathodes comprising either layered or spinel phases [13, 14]. These results indicate that multiphase cathodes in which both layered and spinel phases are integrated, can be cycled in a wide potential range, deliver high specific capacity with prolonged cycling stability, thus improving the energy density of Li-ion batteries. We will present the results of our comparative studies that may allow us to select optimized compositions.

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