Borate Additives in Sodium-Ion Batteries: Cycle Stability

Overview of Technical Issues:

The borate additive insufficiently protects the electrode interface in sodium-ion batteries, failing to prevent interface degradation and electrolyte decomposition during repeated charge-discharge cycles, resulting in capacity fade and poor cycle stability; the goal is to optimize the borate formulation to achieve stable long-term cycling performance with minimal capacity loss.

Solution directions generated for this problem

Problem Direction 1 :

ImproveProtective film formation rate
VS
ConstraintElectrolyte ionic conductivity

Inspiration 1 : Cross-domain reference

Application Principle: #19 Periodic action
Cross-domain applicability Assess applicability
Disconnector closing and opening assisting device and circuit breaker
Innovative Solution Refine solution

Metered borate release microcapsules for fast interphase build

Pulse borate only when needed
How to solve :
  • Disperse borate microcapsules in electrolyte, shell 80–150nm PMMA-co-PEGDA, core 35–45wt% NaDFOB in EC:DEC, dose 0.15–0.30wt%
  • Tune shell to rupture under first 3–5 cycles by 8–12% anode expansion and local reductive swelling, then stop release
  • QC by EIS conductivity 7.0–8.2mS/cm at 25°C, capsule D50 0.8–1.5μm, leakage <5% in 72h, SEI resistance rise <20% after cycle 5
Expected Effect : SEI stable in 3–5 cycles, bulk conductivity >7mS/cm, 500-cycle fade <10%, first-50-cycle loss cut 30–50% vs uniform high-borate dosing
Risk Control :
  • capsule sedimentation
  • premature borate leakage
  • shell residue blocking pores

Problem Direction 2 :

ImproveFilm mechanical strength
VS
ConstraintFormulation synthesis complexity

Inspiration 1 : Cross-domain reference

Application Principle: #27 Cheap short-living objects
Cross-domain applicability Assess applicability
Method for producing a welded steel blank and associated welded blank
Innovative Solution Refine solution

Disposable borate precursor capsule system for robust SEI formation

Replace complex borate with consumable precursor capsules
How to solve :
  • Embed micro-encapsulated simple borate precursors (sodium tetraborate pentahydrate, no moisture-sensitive synthesis) in separator coating at 8-12 mg/cm²
  • capsules rupture during first 3-5 cycles releasing concentrated borate locally at electrode interface
  • Capsule shell made from electrochemically degradable polymer (polyvinylidene fluoride-co-hexafluoropropylene, 0.3-0.6 μm wall thickness) that decomposes at 2.8-3.2V, triggering timed borate release without bulk electrolyte viscosity increase
  • After capsule depletion by cycle 10, the formed boron-oxide reinforced SEI (target thickness 25-40 nm, elastic modulus ≥15 GPa) self-maintains through reversible B-O bond reformation with carbonate decomposition products over 500+ cycles
Expected Effect : Film strength retention >85% at 500 cycles; capacity fade <10%; synthesis steps reduced from 5 to 1; bulk ionic conductivity maintained at 7.5 mS/cm
Risk Control :
  • capsule size distribution uniformity (target CV<15%)
  • premature rupture during assembly
  • incomplete borate release kinetics

Problem Direction 3 :

ImproveInterface protection durability
VS
ConstraintElectrolyte ionic conductivity

Inspiration 1 : Cross-domain reference

Application Principle: #2 Taking out
Cross-domain applicability Assess applicability
Spinel-type lithium-manganese composite oxide
Innovative Solution Refine solution

Electrode-integrated borate reservoir coating for localized SEI protection

Localize protection at electrode interface only
How to solve :
  • Apply borate-enriched polymer coating (15-25 wt% sodium difluoro(oxalato)borate in PVDF matrix, 0.6-1.2 μm thick) directly onto electrode surface before cell assembly via slot-die coating at 60°C
  • The coating acts as localized borate reservoir, releasing borate species only at electrode-electrolyte interface during cycling to continuously repair SEI over 500+ cycles
  • Bulk electrolyte maintains low borate content (≤0.5 wt%), preserving ionic conductivity at 7.5-8.2 mS/cm while interface receives sustained protection from coating reservoir
Expected Effect : Capacity fade <8% over 500 cycles; conductivity maintained ≥7.5 mS/cm; film integrity +400%
Risk Control :
  • coating thickness uniformity control (±0.15 μm tolerance required)
  • borate release kinetics variation with temperature
  • adhesion strength between coating and electrode

Problem Direction 4 :

ImproveInterface protection durability
VS
ConstraintFormulation synthesis complexity

Inspiration 1 : Cross-domain reference

Application Principle: #6 Universality
Cross-domain applicability Assess applicability
Labeled inhibitors of prostate specific membrane antigen (PSMA) as agents for the treatment of prostate cancer
Innovative Solution Refine solution

Single-salt multifunctional borate electrolyte for durable sodium-ion interphases

One borate does both jobs
How to solve :
  • Use NaDFOB as sole functional additive and repair source in 0.8-1.0 M NaPF6 EC:DEC 1:1, additive 1.5-2.5 wt%, water <20 ppm, conductivity acceptance ≥7.0 mS/cm at 25°C by EIS
  • Run 3-step formation at C/20,C/10,C/5 within 2.0-4.0 V and 30-35°C so NaDFOB first reduces to NaF+B-Oxalate rich SEI then continues crack healing, target CE ≥85% first cycle and stable CE ≥99.5% by cycle 5
  • Control SEI consistency by XPS B1s 191.5-193.5 eV and F1s NaF fraction 35-55%, electrode coat weight ±2%, additive assay 98.5-101.5% by 11B NMR, reject cells with Rct rise >15% after 20 cycles
Expected Effect : SEI stable in 3-5 cycles, fade <10%/500 cycles, integrity >500 cycles, conductivity 7.0-7.8 mS/cm, cost and synthesis steps cut >40% vs custom borates
Risk Control :
  • NaDFOB moisture uptake
  • excess salt causing gas
  • SEI too inorganic and brittle
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