Si-Sn Alloy Negative Electrode Amorphous Phase Control
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Solution Overview
Problem
Lithium ion secondary batteries with carbon/graphite-based negative electrodes have limited charge-discharge capacity and cycle durability, while those using silicon-based electrodes offer higher energy density but suffer from poor cycle life due to volume expansion and contraction during charge and discharge.
Innovation Solution
A negative electrode active material composed of a mixture of a predetermined ternary Si alloy and carbon, where the Si alloy is represented by the formula Si x Sn y M z A a, with specific mass percent values for x, y, z, and a, is used to inhibit the amorphous to crystalline phase transition, enhancing cycle life and capacity.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Quantity of substance
If silicon-based negative electrode materials are used, then energy density is improved, but cycle life deteriorates due to volume expansion and contraction
Solution Approach 1:
The patent changes the physical state parameter of silicon from crystalline to amorphous form. This parameter change prevents the phase transition that causes volume expansion and contraction, thereby maintaining cycle life while preserving high energy density. The amorphous structure accommodates lithium alloying reactions without significant volume change.
Solution Approach 2:
The patent creates a composite material system consisting of amorphous silicon alloy particles dispersed in a carbon-containing substance matrix. This composite structure combines the high capacity benefits of silicon with the structural stability and volume compensation properties of carbon, resolving the contradiction between energy density and cycle life.
2Reliability
If carbon/graphite-based negative electrode materials are used, then cycle life is improved, but charge-discharge capacity is limited
Solution Approach 1:
The patent develops a composite negative electrode material combining amorphous silicon alloy with carbon-containing substances. This composite leverages silicon's high theoretical capacity (3200 mAh/g) while using carbon to provide structural stability and prevent volume expansion, achieving both high capacity and long cycle life simultaneously.
Solution Approach 2:
The patent changes the operational mechanism from graphite's intercalation/deintercalation (limited to 372 mAh/g) to silicon's alloying reaction with amorphous structure. This parameter change in the charge-discharge mechanism enables access to silicon's much higher theoretical capacity while the amorphous structure prevents the volume changes that would limit cycle life.
3Quantity of substance
If Si material is used for high capacity, then initial capacity is improved, but volume change during phase transition reduces cycle durability
Solution Approach 1:
The patent changes the crystalline structure parameter of silicon from ordered crystalline phases to disordered amorphous phase. This parameter change eliminates the phase transition between amorphous and crystalline states during lithium alloying, preventing the associated volume expansion and contraction that would otherwise reduce cycle durability. The amorphous structure maintains volume stability while enabling high initial 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 solution provides a negative electrode with high initial capacity and improved cycle durability, achieving balanced performance in lithium ion secondary batteries for vehicle applications by preventing uneven reactions with Li ions and reducing volume changes during alloying.
Implementation Method 1
One mole of a Si material, for example, stores and releases 4.4 mole of lithium ions as expressed by the following reaction formula (A). The theoretical capacity of Li22Si5(=Li44Si) is 2100 mAh/g.
Implementation Method 2
The negative electrodes expand and contract during the processes of charge and discharge. The graphite material expands in volume by about 1.2 times when storing Li ions, for example. On the other hand, the Si material significantly changes in volume (by about 4 times) because Si transits from the amorphous phase to the crystalline phase when Si is alloyed with Li.
Implementation Method 3
the Si material significantly changes in volume (by about 4 times) because Si transits from the amorphous phase to the crystalline phase when Si is alloyed with Li
Data Source
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AI summary
[TECHNICAL PROBLEM] To provide a negative electrode for an electric device, such as a Li ion secondary battery, exhibiting good balanced characteristics of providing initial capacity and maintaining high cycle characteristics. [SOLUTION TO PROBLEM] The negative electrode for an electric device includes: a current collector; and an electrode layer containing a negative electrode active material, an electrically-conductive auxiliary agent and a binder and formed on a surface of the current collector. The negative electrode active material is a mixture of a carbon material and an alloy represented by the following formula (1): SixSnyMzAa ...(1) (in formula (1), M is at least one metal selected from the group consisting of Al, V, C, and combinations thereof, A is inevitable impurity, x, y, z, and a represent mass percent values and satisfy 0 < x < 100, 0 < y < 100, 0 < z < 100, and 0 ≤ a < 0.5, x + y + z + a =100).