Coating - A potent method to enhance electrochemical performance of Li(NixMnyCoz)O2 cathodes for Li-ion batteries

Leon Shaw,Maziar Ashuri

doi:10.5185/amlett.2019.2256

Abstract

Layered lithium nickel manganese cobalt oxides, Li(NixMnyCoz)O2 where x + y + z = 1 (NMCs), have been studied extensively due to their higher capacity, less toxicity and lower cost compared to LiCoO2. However, widespread market penetration of NMCs as cathodes for Li-ion batteries (LIBs) is impeded by their poor capacity retention and low rate capability. Coatings provide an effective solution to these problems. This article focuses on review of the recent advancements in coatings of NMCs from the mechanism viewpoint. This is the first time that coatings on NMCs are reviewed based on their functionalities and mechanisms through which the electrochemical properties and performance of NMCs have been improved. To provide a comprehensive understanding of the functions and mechanisms offered by coatings, the following functions and mechanisms are reviewed individually: (i) scavenging HF in the electrolyte, (ii) scavenging water molecules in the electrolyte and thus suppressing HF propagation during charge/discharge cycles, (iii) serving as a buffer layer to minimize HF attack on NMCs and suppress side reactions between NMCs and the electrolyte, (iv) hindering phase transitions and impeding loss of lattice oxygen, (v) preventing microcracks in NMC particles to keep participation of most NMC material in lithiation/de-lithiation, and (vi) enhancing the rate capability of NMC cathodes. Finally, the personal perspectives on outlook are offered with an aim to stimulate further discussion and ideas on the rational design of coatings for durable and high-performance NMC cathodes for the next generation LIBs in the near future.

Highlights

LiCoO2 is the major cathode material for Li-ion batteries (LIBs) since 1992 because it excels in many electrochemical properties [1]
The loss of the cell capacity during calendar aging is predominately attributed to (i) gradual growth of surface reaction layers at both the anode and cathode which leads to irreversible consumption of cyclable lithium, (ii) transition metal dissolution of NMCs resulting in loss of the active cathode material, (iii) cracking and loosening of NMC particles causing some loss of the active material, and (iv) oxidation of the electrolyte at the cathode leading to an increase in the ohmic resistance of the electrolyte [52,53,54,55]
Volume expansion is serious for Ni-rich NMCs because the unit cell volume expansion of NMCs increases with increasing Ni content at the same number of charge/discharge cycles [8]

Summary

Introduction

LiCoO2 is the major cathode material for Li-ion batteries (LIBs) since 1992 because it excels in many electrochemical properties [1]. The loss of the cell capacity during calendar aging is predominately attributed to (i) gradual growth of surface reaction layers at both the anode and cathode which leads to irreversible consumption of cyclable lithium, (ii) transition metal dissolution of NMCs resulting in loss of the active cathode material, (iii) cracking and loosening of NMC particles causing some loss of the active material, and (iv) oxidation of the electrolyte at the cathode leading to an increase in the ohmic resistance of the electrolyte [52,53,54,55] All of these degradation mechanisms are very similar to those observed during charge/discharge cycles. They all start at the electrode/electrolyte interface and can all be mitigated through appropriate coatings

Methods for forming coatings

Summary and outlook

Findings

Design of coating functionalities