New Surface Treatment Strategy for High-Performance Li-Rich Cathode Material with High Volumetric Energy Density
Authors: Oh Pilgun 
Published: February 2020

Fast development of portable electronic devices (such as a smart phone, iPad, and Galaxy Note) and electrified vehicles (such as EVs and HEVs) require better and smaller lithium ion batteries as their energy storages. What is needed for the cathode of the lithium ion battery is a material capable of a higher volumetric energy density as well as gravimetric energy density. However, not only the development of cathode material but also the commercialization of high energy density anode material has been limited due to cathode material’s much lower energy density compared to anode material. In this regard, Li-rich material (Li2MnO3-LiMO2, M=Ni, Mn and Co) that has 150 % higher energy density than commercialized LiCoO2 is the unique next generation cathode material to solve the problem of cathode material’s low energy density. In the past decade, many researches have focused on the surface stabilization method of this material to improve its intrinsic problems of poor cycle/ rate capability and such surface treatment methods have been demonstrated in a few examples, including AlF3 coating and spinel heterostructures, yielding noticeable improvements in rate and cycle ability.

Although such surface treatments showed improved rate and cycle performances, achieving the high volumetric energy density and long-term cycle life which are more critical factors for the commercialization of Li-rich material still remained unsatisfied. In general, Li-rich material’s reversible capacity is related with its particle size since the Li2MnO3 phase in large Li-rich material, which is a major component to realize high energy density, is not fully activated in low voltage range below 4.6 V. For this reason, many previous papers reported their results with only small primary particle size below 100nm. The high surface area induced by the small particle size increases side reactions with electrolyte resulting poor long-term cycle life as well as lowers the volumetric energy density. Note that Li-rich material’s deteriorations are generated from its surface and continue to affect its cycling capability and the deteriorations can be divided into two mechanisms: one is side reactions with electrolyte, and another is the phase collapse. The side reactions with electrolyte at its high working voltage yield active material attack speiceses such as HF and electrolyte exhaustion causing Li-rich material’s fast discharge capacity decay during cycles. Furthurmore, it is hardly show long-term cycle life with over 200 cycles since the phase transition from layered structure to spinel-like and NiO rock salt is generated on the surface of active material, and followed by expention of this transition into the bulk. For instance, AlF3 coated Li-rich LNCMO has abrupt capacity fade after 100 cycles, although it showed good rate and cycle capability within 100 cycles. Hence, new active material design, rather than simple surface modification methods such as coating or doping, is required to solve above previous limits.

The new material design needs to start with a simple question: how to minimize the damages of surface deteriorations such as side reasction with electrolyte and phase trasnsition on bulk material and maximize volumetric energy density. The most effective solution for the question is increasing the primary particle size with the stable activation method, since the volumetric energy density of cathode material is maximized from synthesis of micron secondary particles consisted of primary particles by using co-precipitation method in industry. Here we demonstrate a novel approach for lithium storage, which is a material design of a secondary structure which consists of large flake shaped primary particle (hundred nanometers x mictrometers) with a novel activation method using simple chemical treatment to achieve superior long-term cycle life with high volumetric energy density. In this design, the large primary particle effectively reduced its surface area producing markedly decreased surface instability reaction as well as high tap density. Interstingly, the chemical approach activated only surface Li2MnO3 phase of large primary particle and the very surface activation effectively overcame the activation problem, which is limit of large primary particle have. This novel concept is very meaningful in that it is the first and unique method to achieve cathode material’s high volumetric energy density with long-term cycle life. As a result, this novel designed material affords remarkable battery performance with extremely high cycle retention during 400 cycles and high volumetric energy density ever reported before.