Passivation, lithium

Kinetic stability of lithium and the lithiated carbons results from film formation which yields protective layers on lithium or on the surfaces of carbonaceous materials, able to conduct lithium ions and to prevent the electrolyte from continuously being reduced film formation at the Li/PC interphase by the reductive decomposition of PC or EC/DMC yielding alkyl-carbonates passivates lithium, in contrast to the situation with DEC where lithium is dissolved to form lithium ethylcarbonate [149]. EMC is superior to DMC as a single solvent, due to better surface film properties at the carbon electrode [151]. However, the quality of films can be increased further by using the mixed solvent EMC/EC, in contrast to the recently proposed solvent methyl propyl carbonate (MPC) which may be used as a single sol-... 

On drawing current from a passivated lithium anode, polarization may be at first severe, but the voltage recovers fairly rapidly (Fig. 4.6). Initially, charge transfer at the anode is limited by lithium ion transport through a thin or imperfect section of the interfacial film. This process progressively... 

The technical basis for the RAPID includes general experience with sodium cooled fast reactors. Specifically, the RAPID concept includes no control rods but incorporates the passive lithium expansion modules, lithium injection modules and lithium release modules to enable an operator-free operation mode. These systems utilize Li as a liquid poison instead of B4C rods. To verify the reactivity worth of Li, the criticality test [XVII-5] using the fast critical assembly (FCA) of the Japan Atomic Research Institute (JAERI) has been conducted. Also, the manufacturing technology of the lithium modules was mastered, and the performance and neutron radiography tests of the lithium expansion and lithium injection module pilots were conducted. 

Unique challenges in the design of reactivity control systems have been addressed in the RAPID concept. The reactor has no control rods but involves the following innovative reactivity control systems [XVII-1 to XVII-6, XVII-9 to XVII-11] passive lithium expansion... 

Owing to the vmeven surface, lithium deposition on passivated lithium may give rise to the formation of dendrites that, by crossing the electrolyte, may reach the cathode, resulting in an electronic shunt with elevated heat associated with the current flow. This in turn can cause the electrolyte to ignite and eventually, the system to explode. 

Liquid Cathode Cells. Liquid cathode cells were discovered at almost the same time as the successful soHd cathode cells. A strongly oxidising hquid such as SO2, was deterrnined to be suitable for direct contact with the strongly reducing lithium, because an excellent passivating film forms... 

This is only possible because lithium forms a passive layer in solutions containing higher amounts of caustic [17, 18]. 

It is now well established that in lithium batteries (including lithium-ion batteries) containing either liquid or polymer electrolytes, the anode is always covered by a passivating layer called the SEI. However, the chemical and electrochemical formation reactions and properties of this layer are as yet not well understood. In this section we discuss the electrode surface and SEI characterizations, film formation reactions (chemical and electrochemical), and other phenomena taking place at the lithium or lithium-alloy anode, and at the Li. C6 anode/electrolyte interface in both liquid and polymer-electrolyte batteries. We focus on the lithium anode but the theoretical considerations are common to all alkali-metal anodes. We address also the initial electrochemical formation steps of the SEI, the role of the solvated-electron rate constant in the selection of SEI-building materials (precursors), and the correlation between SEI properties and battery quality and performance. 

According to the depth profile of lithium passivated in LiAsF6 / dimethoxyethane (DME), the SEI has a bilayer structure containing lithium methoxide, LiOH, Li20, and LiF [21]. The oxide-hydroxide layer is close to the lithium surface and there are solvent-reduction species in the outer part of the film. The thickness of the surface film formed on lithium freshly immersed in LiAsF /DME solutions is of the order of 100 A. 

Among many polar aprotic solvents, including ethers, BL, PC, and ethylene carbonate (EC), methyl formate (MF) seems to be the most reactive towards lithium. It is reduced to lithium formate as a major product which precipitates on the lithium surface and passivates it [24], The presence of trace amounts of the two expected contaminants, water and methanol, in MF solutions does not affect the surface chemistry. C02 in MF causes the formation of a passive film containing both lithium formate and lithium carbonate. 

It was concluded [93, 94J that, on long cycling of the lithium-ion battery, the passivating layer on the carbon anode becomes thicker and more resistive, and is responsible, in part, for capacity loss. 

Since this is a new field, little has been published on the LiXC6 /electrolyte interface. However, there is much similarity between the SEIs on lithium and on LixC6 electrodes. The mechanism of formation of the passivation film at the interface between lithiated carbon and a liquid or polymer electrolyte was studied by AC impedance [128, 142]. Two semicircles observed in AC-impedance spectra of LiAsF6/EC-2Me-THF electrolytes at 0.8 V vs. Li/Li+ [142] were attributed to the formation of a surface film during the first charge cycle. However, in the cases of LiC104 or LiBF4 /EC-PC-DME (di-... 

Figure 9. CV of 0.2 mol kg 1 lithium bis[2,2 -biphenyldiolato(2-)-0,0 ]borate solution in PC at a stainless steel electrode, area 0.5 cm 2, showing the passivation of the electrode.

Subsequent deposition and dissolution of lithium at the anodically passivated electrode by CV in the range -1000 mV to 6000 mV versus Li was successful, showing that the passivating film at the electrode is only impermeable for the anions, but not for lithium ions however, the amount of lithium deposited decreases, with cycle number. 

Upon an increase of the anodic reverse potential finally up to 8 V versus Li the cyclic voltammogran corresponding to Fig. 9 remains unchanged, showing that the passivating layer at the electrode also protects the solvents (PC and DME) from being oxidized. Subsequent deposition and dissolution of lithium at the passivated electrodes remains possible when the electrode is passivated but the cycling efficiency decreases. 

CV of solutions of lithium bis[ salicy-lato(2-)]borate in PC shows mainly the same oxidation behavior as with lithium bis[2,2 biphenyldiolato(2-)-0,0 ] borate, i.e., electrode (stainless steel or Au) passivation. The anodic oxidation limit is the highest of all borates investigated by us so far, namely 4.5 V versus Li. However, in contrast to lithium bis[2,2 -biphenyl-diolato(2-)-0,0 Jborate based solutions, lithium deposition and dissolution without previous protective film formation by oxidation of the anion is not possible, as the anion itself is probably reduced at potentials of 620-670 mV versus Li, where a... 

Passivating films, which are formed in less than a second on the surface when lithium is exposed to a suitable solution determine [153]... 

Passivating films may change their chemical composition after their formation due to reactions with water or carbon dioxide lithium alkylcarbonates react with traces of water to yield lithium carbonate (see Table 8). 

With regard to rechargeable cells, a number of laboratory studies have assessed the applicability of the rocking-chair concept to PAN-EC/PC electrolytes with various anode/cathode electrode couples [121-123], Performance studies on cells of the type Li°l PAN-EC/PC-based electrolyte lLiMn20 and carbon I PAN-EC/PC-based electrolyte ILiNi02 show some capacity decline with cycling [121]. For cells with a lithium anode, the capacity decay can be attributed mainly to passivation and loss of lithium by its reaction with... 

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