Silicon-Carbon Composite Anode with Mg Constraint for Cycle Retention
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Solution Overview
Problem
Existing silicon-based negative electrode active materials for lithium secondary batteries face challenges due to significant volume expansion and contraction during charging and discharging, leading to structural degradation and reduced capacity retention rates.
Innovation Solution
A silicon-carbon composite is developed, comprising silicon particles, a magnesium compound, and carbon, with a specific molar ratio of oxygen to silicon atoms (O/Si) ranging from 0.01 to 0.60, and a core-shell structure with carbon layers, to enhance mechanical properties and stability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Quantity of substance
If silicon-based negative electrode active materials are used to enhance battery capacity, then the theoretical capacity increases significantly (4,199 mAh/g vs 372 mAh/g for graphite), but volume expansion of up to 300% occurs during lithium intercalation, destroying the negative electrode structure
Solution Approach 1:
Silicon particles are encapsulated within carbon particles, forming a core-shell structure where the silicon is nested inside the carbon matrix. This nesting approach allows the high-capacity silicon to be protected from structural degradation while maintaining its electrochemical activity, resolving the contradiction between capacity enhancement and structural stability
Solution Approach 2:
A composite material system is created combining silicon particles with carbon particles (including fine carbon particles and coarse-grained carbon particles). The composite structure leverages the high capacity of silicon while the carbon matrix provides structural stability and conductivity, preventing the 300% volume expansion from destroying the electrode
2Quantity of substance
If silicon particles undergo volume expansion during lithium intercalation, then high capacity is achieved, but cracks form on the surface and ionic material is formed inside, causing electrical detachment from the current collector
Solution Approach 1:
A carbon layer is formed on the surface of the silicon-containing particles, creating a flexible shell that can accommodate volume changes during lithium intercalation. This carbon shell prevents crack formation and maintains electrical conductivity, ensuring the negative electrode remains electrically connected to the current collector even during significant volume expansion
Solution Approach 2:
The carbon matrix acts as an intermediary between the silicon particles and the current collector. It provides a stable conductive network that maintains electrical connection even when silicon particles expand and contract, preventing direct mechanical stress transmission that would cause detachment
3Stability of the object's composition
If existing silicon-carbon composites are used to suppress volume expansion, then some structural stability is achieved, but the suppression effect is insufficient and capacity retention rate remains poor
Solution Approach 1:
Different regions of the composite particle have different properties: the core contains silicon particles for high capacity, the intermediate layer contains fine carbon particles for volume expansion suppression, and the outer layer contains coarse-grained carbon particles for structural stability and conductivity. This local differentiation of properties achieves both volume control and high capacity retention
Solution Approach 2:
The invention optimizes specific parameters including the size distribution of carbon particles (fine and coarse-grained), the ratio of silicon to carbon, and the thickness of the carbon shell. By carefully controlling these parameters, the composite achieves optimal balance between volume expansion suppression and capacity retention, overcoming the limitations of existing composites
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 composite improves discharge capacity, initial efficiency, and capacity retention rate of lithium secondary batteries by minimizing volume expansion and maintaining electrical conductivity.
Implementation Method 1
a magnesium compound... having a molar ratio (O/Si) of oxygen (O) atoms to silicon (Si) atoms in the silicon-carbon composite that satisfies a specific range... having excellent capacity retention rate, along with significantly improved discharge capacity and initial efficiency
Implementation Method 2
comprising silicon particles, a magnesium compound, and carbon... maintaining electrical conductivity
Implementation Method 3
a core-shell structure with carbon layers, to enhance mechanical properties and stability
Data Source
AI summary
The present invention relates to a silicon-carbon composite, a preparation method therefor, and an anode active material comprising same. The silicon-carbon composite includes silicon particles, silicon oxides, magnesium compounds, and carbon. As a molar ratio (O/Si) of oxygen (O) atoms to silicon (Si) atoms in the silicon-carbon composite satisfies 0.01 to 0.60, when the silicon-carbon composite is applied to an anode active material, the discharge capacity, initial efficiency, and capacity retention ratio after cycles of a lithium secondary battery may be simultaneously improved.