Li-rich Cathode Material with Phosphate Surface Layer for Cycle Stability
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Solution Overview
Problem
Lithium ion secondary batteries face challenges in achieving excellent cycle characteristics and minimizing discharge voltage decrease, particularly with Li-rich cathode active materials that are prone to manganese elution and structural instability.
Innovation Solution
A process involving the mixing of sulfates and carbonates to form a coprecipitated compound, followed by phosphate treatment and firing with lithium carbonate, to produce a cathode active material with specific composition and structural properties that enhance cycle stability and discharge voltage retention.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Quantity of substance
If a Li-rich cathode active material with high discharge capacity is used, then the discharge capacity is improved, but the cycle characteristics deteriorate due to manganese elution and crystal structure instability
Solution Approach 1:
The invention applies local quality by creating a dual-structure cathode material where the interior maintains high Li content (Li1.2Co0.1Mn0.6O2) for high discharge capacity, while the surface is modified with a phosphate-containing layer (Li1-xCoxPO4) to suppress manganese elution and improve cycle characteristics. This local differentiation of composition and function resolves the contradiction between high capacity and good cycle stability.
Solution Approach 2:
The invention uses composite materials by combining Li-rich layered oxide (Li1.2Co0.1Mn0.6O2) with a phosphate-containing surface layer (Li1-xCoxPO4) to create a composite cathode active material. The composite structure integrates the high capacity advantage of Li-rich materials with the structural stability and manganese-elution resistance of phosphate compounds, thereby resolving the technical contradiction.
2Quantity of substance
If the Li ratio is increased to achieve high discharge capacity, then the discharge capacity is improved, but the crystal structure stability deteriorates
Solution Approach 1:
The high Li ratio (Li1.2) is maintained in the interior of the cathode particles to ensure high discharge capacity, while the surface is locally modified with a phosphate-containing layer that provides structural stability. This spatial separation of functions allows the bulk material to maximize capacity while the surface layer maintains crystal structure stability during cycling.
Solution Approach 2:
The phosphate-containing surface layer is formed through preliminary action by contacting the Li-rich cathode material with a lithium dihydrogen phosphate solution before battery assembly. This pre-formed protective layer prevents crystal structure degradation and manganese elution during subsequent charge-discharge cycles, enabling the high Li ratio material to maintain stability.
3Reliability
If conventional phosphate treatment methods are used on Li-rich cathode materials, then surface protection is attempted, but sufficient cycle characteristics are not achieved
Solution Approach 1:
The invention optimizes parameters including the concentration of lithium dihydrogen phosphate solution (0.1-5 mol/L), treatment temperature (20-80°C), and treatment time (1-24 hours) to achieve uniform phosphate incorporation. By carefully controlling these parameters, the method ensures sufficient phosphate is incorporated into the surface layer to improve cycle characteristics, overcoming the limitations of conventional treatment methods.
Solution Approach 2:
The phosphate treatment creates a localized surface modification rather than bulk changes. The phosphate-containing layer forms specifically at the particle surfaces where manganese elution occurs, providing targeted protection without altering the high-capacity Li-rich composition in the interior. This localized approach achieves both improved cycle characteristics and maintained high discharge capacity.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The resulting cathode active material exhibits improved cycle characteristics and reduced discharge voltage decrease, leading to higher discharge capacity and retention in lithium ion secondary batteries.
Implementation Method 1
a step of mixing at least one sulfate (A) selected from the group consisting of sulfate of Ni, a sulfate of Co and a sulfate of Mn, with at least one carbonate (B) selected from the group consisting of sodium carbonate and potassium carbonate in an aqueous solution state to obtain a coprecipitated compound containing at least one transition metal element (X)
Implementation Method 2
a step of volatilizing a water content from the mixture of the coprecipitated compound and the aqueous phosphate solution to obtain a precursor compound
Implementation Method 3
a step of mixing the precursor compound with lithium carbonate and firing the mixture at from 500 to 1000° C.
Data Source
AI summary
To provide a cathode active material having excellent cycle characteristics and a small decrease in the discharge voltage, and a process for its production.A process for producing a cathode active material, which comprises a step of mixing at least one sulfate (A) selected from the group consisting of a sulfate of Ni, a sulfate of Co and a sulfate of Mn with at least one carbonate (B) selected from the group consisting of sodium carbonate and potassium carbonate in an aqueous solution state to obtain a coprecipitated compound, a step of mixing the coprecipitated compound with an aqueous phosphate solution, a step of volatilizing a water content from the mixture of the coprecipitated compound and the aqueous phosphate solution to obtain a precursor compound, and a step of mixing the precursor compound with lithium carbonate and firing the mixture at from 500 to 1000° C.; and a cathode active material obtainable by the production process, which comprises Li, at least one transition metal element (X) selected from the group consisting of Ni, Co and Mn, and P, wherein the average coefficient of variation (CV value) of the calculated peak intensity ratio (Ip/Ix) of P to the transition metal element (X) is from 0 to 20% as determined by a method for measuring coefficient of variation.