Si Alloy Negative Electrode for High Cycle Durability
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Solution Overview
Problem
Lithium ion secondary batteries for vehicle applications face challenges in achieving high cycle durability and energy density due to the limitations of carbon graphite-based negative electrodes, which suffer from low capacity and rapid degradation caused by lithium ion absorption and desorption.
Innovation Solution
A ternary Si alloy with specific compositions, including Si, Sn, Ti, Zn, or C, is used as the negative electrode active material, with a half-width of the (111) diffraction peak in the range of 2θ=24 to 33° by X-ray diffraction, minimizing amorphous-crystal phase transitions and enhancing cycle life.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If carbon graphite-based material is used for negative electrode, then charge and discharge cycle life is improved, but capacity is limited to theoretical maximum of 372 mAh/g
Solution Approach 1:
The invention changes the material composition parameter from pure carbon graphite to a composite containing Si alloy particles (30-70 wt%) combined with carbon graphite and binder. This parameter change enables the negative electrode to achieve capacity exceeding the theoretical limit of graphite (372 mAh/g) by utilizing Si's alloying mechanism with lithium, while the carbon graphite component maintains structural stability for good cycle life.
Solution Approach 2:
The invention creates a composite negative electrode material consisting of three main components: Si alloy particles (providing high capacity through alloying), carbon graphite (providing structural stability and conductivity), and binder (holding components together). This composite structure combines the advantages of both materials to achieve high capacity while maintaining acceptable cycle life.
2Quantity of substance
If Si material is used for negative electrode, then energy density is improved with theoretical capacity of 2100 mAh/g, but volume expansion of about 4 times occurs during alloying
Solution Approach 1:
The invention applies local quality by creating a heterogeneous structure where Si alloy particles (providing high energy density) are distributed within a carbon graphite matrix (providing volume stability). Each component performs its specialized function: Si alloy contributes to high capacity while carbon graphite constrains volume expansion locally at each particle interface, preventing catastrophic structural failure.
Solution Approach 2:
The carbon graphite component acts as an intermediary between Si alloy particles and the electrolyte, and also as a structural mediator that constrains Si alloy volume expansion. The binder serves as another intermediary that holds the composite structure together, distributing mechanical stress and maintaining electrode integrity during cycling.
3Quantity of substance
If Si negative electrode active material is used, then capacity is increased, but cycle durability deteriorates due to large volume change
Solution Approach 1:
The composite structure combines Si alloy (high capacity) with carbon graphite (structural stability) and binder (cohesion). The carbon graphite matrix and binder work together to maintain electrode integrity during repeated volume changes, preventing particle detachment and structural collapse that would otherwise occur with pure Si, thereby improving cycle durability.
Solution Approach 2:
The carbon graphite component and binder provide beforehand cushioning by creating a flexible, stress-absorbing matrix that accommodates Si alloy volume expansion before it can cause structural damage. This pre-configured cushioning structure prevents catastrophic failure during subsequent cycling by distributing mechanical stress throughout the composite.
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 Si alloy-based negative electrode material achieves higher capacity and significantly improved cycle durability, retaining discharge capacity retention rates of over 90% after 50 cycles and maintaining performance even after 100 cycles, outperforming traditional carbon-based materials.
Implementation Method 1
a material capable of forming an alloy with Li is expected as a negative electrode material for vehicle use
Implementation Method 2
Si material exhibits absorption and desorption of lithium ions in an amount of 4.4 mol per mol
Implementation Method 3
the graphite material shows volume expansion of about 1.2 times by absorption of Li ions, the Si material exhibits a large volume change (about 4 times)
Implementation Method 4
a half-width of a (111) diffraction peak in a range of 2θ＝24 to 33° by X-ray diffraction
Implementation Method 5
a half-width of the (111) diffraction peak in the range of 2θ=24 to 33° by X-ray diffraction
Data Source
AI summary
A negative electrode active material having high cycle durability contains an alloy represented by the following chemical formula (1):SixSnyMzAa (1)wherein M is Zn, A is unavoidable impurities, x, y, z, and a represent % by mass values, and in that case, 0<x<100, 0<y<100, 0<z<100, 0≤a<0.5, and x+y+z+a=100, in which the half width of the diffraction peak of the (111) surface of Si in the range of 2θ=24 to 33° by X ray diffraction measurement of the alloy using CuKα ray is 0.7° or more, and the x is more than 23 and less than 64, the y is 4 or more and less than 34, and the z is more than 0 and less than 65.