Electrochemical recovery of lithium salts from spent electrolytes presents a promising approach to address the growing demand for lithium in battery production while mitigating environmental concerns associated with disposal. The process leverages selective electrodeposition and decomposition mechanisms to reclaim valuable components, particularly lithium hexafluorophosphate (LiPF6) and lithium ions, from complex electrolyte mixtures. This analysis examines the critical aspects of electrochemical lithium salt recovery, including electrode materials, cell configurations, reaction mechanisms, and operational challenges.

Electrode materials play a pivotal role in determining the efficiency and selectivity of lithium recovery. Platinized titanium electrodes exhibit high stability in aggressive electrolyte environments, with reported corrosion resistance exceeding 500 hours in concentrated LiPF6 solutions. The platinum coating provides a low overpotential for target reactions while minimizing parasitic processes. Boron-doped diamond electrodes offer superior electrochemical windows, typically ranging up to 3.5 V in organic electrolytes, enabling selective oxidation of PF6- anions without electrode degradation. Comparative studies indicate boron-doped diamond achieves 15-20% higher current efficiency for PF6- decomposition compared to conventional carbon electrodes.

Cell design significantly influences the recovery process efficiency. Divided cell configurations, employing ion-exchange membranes, demonstrate lithium recovery rates of 85-92% by preventing recombination of decomposition products. The membrane selectivity, typically measured by transport numbers above 0.9 for Li+, ensures efficient ion separation. Undivided cells, while simpler in construction, show reduced lithium recovery efficiency of 60-75% due to side reactions between electrodes and electrolyte components. The choice between designs involves tradeoffs between energy consumption, with divided cells requiring 3.5-4.2 kWh/kg Li versus 2.8-3.3 kWh/kg Li for undivided systems, and product purity.

Potential control strategies optimize the recovery process by targeting specific redox reactions. Constant potential electrolysis at 4.1-4.3 V versus Li/Li+ selectively decomposes PF6- anions while minimizing solvent degradation. Pulse potential techniques, employing alternating high and low potentials, improve lithium deposition morphology, reducing dendritic growth by 40-50% compared to constant potential methods. Advanced potential sweep methods, such as linear sweep voltammetry coupled with potentiostatic holds, enhance the separation efficiency of lithium salts from additive decomposition products.

The reaction mechanism for LiPF6 decomposition proceeds through several electrochemical steps. Initial oxidation of PF6- occurs at the anode, yielding phosphorus pentafluoride (PF5) and fluorine radicals. The PF5 subsequently reacts with trace water to form hydrogen fluoride (HF) and phosphorus oxyfluoride (POF3). Simultaneously, lithium ions migrate to the cathode, where reduction yields metallic lithium or lithium hydroxide depending on the solvent system. Studies using in situ Fourier-transform infrared spectroscopy confirm these reaction pathways, with Faradaic efficiencies reaching 78-85% for complete LiPF6 decomposition.

Lithium redeposition mechanisms vary based on solvent composition and operating conditions. In carbonate-based electrolytes, lithium deposits as a dense metal layer at current densities below 5 mA/cm2, with coulombic efficiencies of 90-94%. Higher current densities induce dendritic growth, reducing efficiency to 70-80%. Additive-containing systems, such as those with vinylene carbonate or fluoroethylene carbonate, modify deposition morphology through SEI layer formation, improving cycling stability by 30-40%.

Current efficiency and energy consumption metrics demonstrate the process viability. Laboratory-scale systems achieve lithium recovery current efficiencies of 80-88%, with energy consumption ranging from 3.2-4.0 kWh per kilogram of recovered lithium. Pilot-scale implementations report slightly lower efficiencies of 75-82% due to scaling effects, with energy consumption increasing to 4.2-4.8 kWh/kg. These values compare favorably with conventional hydrometallurgical lithium recovery, which typically requires 6-8 kWh/kg.

Electrolyte additives present significant challenges for selective lithium recovery. Common additives like vinylene carbonate and lithium bis(oxalato)borate decompose at potentials overlapping with lithium deposition, reducing recovery efficiency by 10-15%. Multi-step potential protocols can mitigate this interference, with studies demonstrating 85% additive removal prior to lithium recovery. Fluorinated additives pose particular difficulties due to their similar decomposition products to LiPF6, requiring precise potential control within ±50 mV to maintain selectivity.

Side reactions constitute another major challenge in electrochemical lithium recovery. Solvent decomposition, particularly of ethylene carbonate and dimethyl carbonate, generates gaseous byproducts that reduce current efficiency by 5-10%. Acidic decomposition products, primarily HF, accelerate electrode corrosion and require neutralization strategies. Advanced cell designs incorporating in situ pH control maintain stable operation for over 100 cycles, compared to 30-40 cycles in uncontrolled systems.

The presence of transition metal impurities, typically below 50 ppm in spent electrolytes, further complicates the recovery process. Nickel and cobalt ions codeposit with lithium at potentials only 0.1-0.2 V more positive, requiring precise potential control or preliminary purification steps. Chelating agents like EDTA can suppress metal deposition, improving lithium purity from 95% to 99.5% in optimized systems.

Scaling electrochemical lithium recovery faces several technical hurdles. Electrode fouling from decomposition products reduces active surface area by 15-20% per 50 cycles, necessitating periodic regeneration. Membrane degradation in divided cells limits operational lifetimes to 200-300 cycles before replacement. Continuous systems demonstrate better scalability than batch processes, with prototype designs achieving 80% of laboratory performance at tenfold increased throughput.

Future developments aim to address these limitations through material innovations and process optimization. Novel electrode coatings combining platinum with rare earth oxides show promise in reducing fouling rates by 30-40%. Advanced membrane materials incorporating graphene oxide layers exhibit improved chemical stability, with preliminary tests showing less than 5% degradation after 500 cycles. Process intensification through combined electrochemical and physical separation methods could further enhance recovery rates while reducing energy consumption.

The electrochemical approach to lithium salt recovery offers a sustainable alternative to conventional recycling methods, with the potential to recover over 90% of lithium from spent electrolytes. Continued refinement of electrode materials, cell designs, and operating protocols will be essential to achieve industrial-scale viability. As battery production volumes increase exponentially, such advanced recovery technologies will play a crucial role in establishing a circular economy for critical battery materials.