Core-Shell Lithium Cathode Material to Limit Voltage Decay
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Solution Overview
Problem
Existing lithium-nickel-manganese-cobalt oxide cathode active materials suffer from unsatisfactory rate capability, poor cycle life characteristics, and voltage decay due to phase transition during life cycling, which hinders their practical application in lithium secondary batteries.
Innovation Solution
A cathode active material with a lithium composite oxide having a layered structure of overlithiated oxide, featuring a core-shell structure with varying crystal structures and composition gradients, is developed to suppress phase transition and enhance lithium ion mobility and structural stability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Quantity of substance
If overlithiated layered oxide is applied to increase reversible capacity, then charge/discharge capacity is improved, but phase transition occurs during life cycling causing decreased cycle life and voltage decay
Solution Approach 1:
The patent applies local quality by creating a core-shell structure where the surface region has different crystal structure composition (higher R-3m phase content) compared to the core. This local differentiation allows the surface to resist phase transition while the core maintains high capacity, thus resolving the contradiction between improved capacity and maintained cycle life.
Solution Approach 2:
The patent uses composite materials by combining different crystal phases (C2/m and R-3m) within the same particle structure. The composite nature of having both phases, with the R-3m phase enriched at the surface, provides both high reversible capacity and resistance to phase transition during cycling.
2Ease of manufacture
If conventional Li(NixCoyMnz)O2 is used, then cost is reduced due to small Co content, but rate capability and cycle life characteristics at high temperatures are poor
Solution Approach 1:
The patent applies parameter changes by modifying the crystal structure phase composition (increasing R-3m phase content at the surface) and lithium content (overlithiation) to improve rate capability and cycle life while maintaining cost-effectiveness through continued use of low-cobalt composition.
3Stability of the object's composition
If phase transition is suppressed to increase charge/discharge capacity, then structural stability is improved, but lithium ion mobility must be enhanced
Solution Approach 1:
The patent applies local quality by creating distinct functional regions: the core maintains structural stability while the surface region, with higher R-3m phase content, provides enhanced lithium ion mobility. This local differentiation allows simultaneous achievement of structural stability and improved ion transport.
Data Source
AI summary
A positive electrode active material for a secondary battery comprises a lithium composite oxide represented by chemical formula 1 below and containing a layer-structured lithium excess oxide, wherein the lithium composite oxide comprises a secondary particle; the secondary particle comprises at least one primary particle; the primary particle comprises at least one crystallite; at least any one of the secondary particle, the primary particle, or the crystallite comprises a core and a shell occupying at least a portion of the surface of the core. When the crystal structure of space group C2/m is designated as [C2/m], the crystal structure of space group R-3m is designated as [R-3m], and the ratio of the crystal structure of space group C2/m versus the crystal structure of space group R-3m is designated as [C2/m]/[R-3m], the [C2/m]/[R-3m] of the core differs from that of the shell in the secondary particle: [chemical formula 1] rLi2M1O3·(1−r)LiaM2O2.