Cathode Active Substance Porosity Control for Internal Resistance
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Solution Overview
Problem
Lithium ion secondary batteries experience an increase in internal resistance due to charge/discharge cycles, primarily caused by the ease of forming contact surfaces with the electrolyte solution, leading to Li deficiency and increased contact area, which accelerates resistance growth.
Innovation Solution
A cathode active substance with a lithium composite compound represented by Li1+αNixCoyM11-x-y-zM2zO2+β, where α, β, x, y, and z are within specific ranges, and M1 includes Mn or Al, M2 includes Mg, Ti, Zr, Mo, or Nb, with a porosity ratio of 1.5 or less, to stabilize the layered structure and reduce contact area with the electrolyte.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Use of energy by moving object
If the cathode active substance uses a high-nickel content composition to increase charge/discharge capacity, then energy density is improved, but internal resistance increases due to Li deficiency on primary particle surfaces
Solution Approach 1:
The patent applies local quality by creating a core-shell structure where the inner core contains high-nickel content material for high capacity, while the outer shell contains protective material that prevents Li deficiency and stabilizes the surface. This allows different regions of the particle to have different compositions optimized for their specific functions.
Solution Approach 2:
The patent uses composite materials by combining high-nickel cathode active substance with protective coating materials to form a composite structure. The composite nature allows the benefits of high-nickel materials (high capacity) while mitigating their drawbacks (surface instability, Li deficiency) through the protective shell.
2Speed
If the cathode active substance particles are pressed at high pressure (100 MPa) to reduce particle size ratio, then rate property is improved, but contact surface area with electrolyte increases leading to faster Li deficiency
Solution Approach 1:
The patent employs a protective shell structure that acts as a flexible barrier between the primary particles and the electrolyte. This shell maintains particle integrity during pressing operations while controlling the contact surface area, preventing excessive electrolyte access that would cause Li deficiency.
Solution Approach 2:
The protective coating is applied in advance before the pressing operation. This preliminary action prepares the particle surface to withstand the subsequent high-pressure pressing without generating excessive contact surface area, thereby preventing Li deficiency from the outset.
3Productivity
If the porosity ratio (Pp/Pi) is increased to improve electrolyte penetration, then charge/discharge performance is enhanced, but Li deficiency on surface increases accelerating internal resistance
Solution Approach 1:
The patent utilizes controlled porosity within the protective shell structure, creating a hierarchical pore system that allows electrolyte penetration to reach active material while maintaining overall particle integrity. The porosity is optimized to balance electrolyte access with surface protection.
Solution Approach 2:
The protective shell exhibits local quality variations with different porosity levels - more porous regions allow electrolyte access while denser regions provide protection. This spatial variation in structure optimizes both electrolyte penetration and surface stability.
Data Source
AI summary
Provided are a cathode active substance used for a lithium ion secondary battery capable of suppressing an increase in an internal resistance inside the battery caused following charge/discharge cycles, a cathode including the cathode active substance, and a lithium ion secondary battery provided with the cathode. The cathode active substance includes a lithium composite compound represented by Formula: Li1+αNixCoyM11-x-y-zM2zO2+β. When Pi is defined as porosity with respect to an opening diameter of 0.6 μm or less and measured by subjecting the active substance to a mercury press-in method, and Pp is defined as porosity with respect to the same diameter and measured by filling the active substance in a mold with an inner diameter of 10 mm, pressing the filled substance by a load of 40 MPa, and subjecting the pressed substance to the same method, a value of Pp/Pi is 1.5 or less.