Lithium Iron Phosphate Electrode Oxidation Control
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Solution Overview
Problem
Lithium iron phosphate batteries face challenges with oxidation of Fe(II) to Fe(III) due to oxygen, leading to structural changes and difficulty in controlling the thickness of the NASICON structure, which affects the reliability and cell potential of power storage devices, and the use of lithium in negative electrodes complicates the manufacturing process.
Innovation Solution
A power storage device with a positive electrode comprising a collector and films of lithium iron phosphate with specific composition ratios, where one film contains Fe(II) and the other Fe(III), formed using a sputtering method with controlled oxygen partial pressures to prevent oxidation and ensure stable battery characteristics, and a lithium-free negative electrode is used.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Power
If heat treatment is performed on amorphous lithium iron phosphate to form crystalline structure, then battery capacitance and discharge potential are improved, but Fe(II) is oxidized to Fe(III) by residual oxygen forming unwanted NASICON structure
Solution Approach 1:
The patent applies inert atmosphere by performing heat treatment in a nitrogen atmosphere instead of air or oxygen-containing environment. This prevents oxidation of Fe(II) to Fe(III) during the heat treatment process, allowing the amorphous lithium iron phosphate to transform into desired crystalline structures (olivine LiFePO4 or spinel LiFe2O4) without forming unwanted NASICON phase. The nitrogen atmosphere serves as an inert environment that maintains the reduction state of iron while enabling crystallization.
Solution Approach 2:
The patent utilizes parameter changes by controlling the oxygen partial pressure during heat treatment. By maintaining low oxygen partial pressure (inert atmosphere) and adjusting temperature parameters (heating to specific temperature ranges), the patent achieves controlled transformation from amorphous to crystalline structure while preventing unwanted oxidation. This parameter control allows selective formation of desired crystal phases with specific Fe oxidation states.
2Stability of the object's composition
If lithium iron phosphate with NASICON structure containing Fe(III) is used as positive electrode, then structural stability is improved, but Li ions must be inserted requiring lithium-containing negative electrode which complicates manufacturing
Solution Approach 1:
The patent inverts the conventional approach by controlling the oxidation state during heat treatment to prevent formation of Fe(III)-rich NASICON structure. Instead of accepting Fe(III) formation and compensating with lithium-containing negative electrodes, the patent uses inert atmosphere heat treatment to maintain Fe(II) state, thereby enabling use of standard graphite negative electrodes. This inversion of the oxidation control approach simplifies the overall battery design.
3Ease of manufacture
If amorphous lithium iron phosphate is used without heat treatment, then manufacturing process is simplified, but natural oxidation by air occurs forming NASICON structure with uncontrolled thickness
Solution Approach 1:
The patent applies preliminary action by performing heat treatment in inert atmosphere before the battery assembly is completed and before any exposure to air. This preliminary heat treatment in controlled nitrogen atmosphere establishes the desired crystalline structure and Fe oxidation state before the material is exposed to air during subsequent handling. This timing of the heat treatment step prevents subsequent oxidation that would occur if the material were exposed to air after formation.
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
This approach enables the manufacture of power storage devices with high reliability and favorable battery characteristics by preventing oxidation and maintaining stable electrode structures, simplifying the manufacturing process and improving cell potential.
Implementation Method 1
formed by a technique for forming a thin film (e.g., a sputtering method)
Implementation Method 2
when subjected to heat treatment, lithium iron phosphate having an amorphous structure can be changed into lithium iron phosphate having a crystalline structure
Implementation Method 3
Fe(II) is readily oxidized to Fe(III) by oxygen in an atmosphere. For example, in a step of performing heat treatment on lithium iron phosphate having an amorphous structure which is formed by a technique for forming a thin film, residual oxygen in a heat treatment atmosphere oxidizes Fe(II)
Implementation Method 4
a redox reaction occurs in iron due to insertion and extraction of a lithium ion. For example, it is known that in LiFePO4 having an olivine structure, a redox reaction between Fe(II) and Fe(III) occurs due to insertion and extraction of a lithium ion
Data Source
AI summary
A power storage device with favorable battery characteristics and a manufacturing method thereof are provided. The power storage device includes at least a positive electrode and a negative electrode provided so as to face the positive electrode with an electrolyte provided therebetween. The positive electrode includes a collector and a film containing an active material over the collector. The film containing the active material contains LieFefPgOh satisfying relations 3.5≦h/g≦4.5, 0.6≦g/f≦1.1, and 0≦e/f≦1.3 and LiaFebPcOd satisfying relations 3.5≦d/c≦4.5, 0.6≦c/b≦1.8, and 0.7≦a/b≦2.8. The film containing the active material contains the LiaFebPcOd satisfying the relations 3.5≦d/c≦4.5, 0.6≦c/b≦1.8, and 0.7≦a/b≦2.8 in a region which is in contact with the electrolyte.