Page 113 - Handbook of Battery Materials

2.8 Secondary Lithium Batteries with Metal Anodes 79

The system can prevent explosion, fire, and venting with fire under
conditions of abuse. These batteries have a unique battery chemistry based on
LiAsF 6 /1,3-dioxolane/tributylamine electrolyte solutions which provide internal
safety mechanism that protect the batteries from short-circuit, overcharge, and
◦
thermal runaway upon heating to 135 C. This behavior is due to the fact that the
electrolyte solution is stable at low-to-medium temperatures but polymerizes at a
◦
temperature over 125 C [87].
The active material of the negative electrode is lithium metal, and that of the
positive electrode is amorphous V 2 O 5 (a–V 2 O 5 ). A prototype AA-size battery has
−1
an energy of 2 Wh (900 mAh), an energy density of 110 Wh kg −1 or 250 Wh L ,
and a life of 150–300 cycles depending on the discharge and charge currents.
One of the most important factors determining whether or not secondary
lithium metal batteries become commercially viable is battery safety, which is
affected by many factors: insufficient information is available about safety of
practical secondary lithium metal batteries [88]. Vanadium compounds dissolve
electrochemically and are deposited on the lithium anode during charge–discharge
cycle. The low reactivity of the vanadium-deposited lithium anode has been observed
by calorimetry; a chemical-state analysis and morphological investigation of the
lithium anode suggest that the improvement in stability is primarily due to a
passivation film [89].
Films on lithium play an important part in secondary lithium metal batteries.
Electrolytes, electrolyte additives, and lithium surface treatments modify the lithium
surface and change the morphology of the lithium and its current efficiency [90].
Various cyclic ethers are reported to be superior solvents for secondary lithium
metalbatteries.1,3-Dioxolane [91,92]and DME[92]showgood cycliccharacteristics.
1,3-Dioxolane-LiB (CH 3 ) 4 is highly conductive and has shown utility as an electrolyte
in ambient temperature secondary lithium battery systems wherein a high rate of
current drain is a desirable feature [93]. Researchers at Exxon used 1,3-dioxolane
or DME-LiClO 4 or LiB(C 6 H 5 ) 4 and 2-methyltetrahydrofuran-LiAsF 6 (2MeTHF) in
a lithium-titanium disulfide system [94]. 1,3-Dioxolane-DME-Li 2 B 10 Cl 10 exhibited
chemical stability toward the components of a lithium-titanium disulfide cell
and showed promise as an electrolyte in such cells [95]. Among various systems
composed of an ether-based solvent and a lithium salt, THF-LiAsF 6 was the least
reactive to lithium at elevated temperature and gave the best cycling efficiency
[96, 97]. THF-diethyl ether-LiAsF 6 afforded lithium electrode cycling efficiency in
excess of 98% [98].
2MeTHF showed good cycling characteristics [99–101], and 2MeTHF–LiAsF 6
showed promise of yielding high energy density and cycle life [102]. In an
investigation of THFs methylated in the α-position: 2MeTHF-LiAsF 6 and
2,5-dimethyltetrahydrofuran–LiAsF 6 showed good cycling characteristics [103].
Several solvents other than ethers have also been reported to be superior
solvents for secondary lithium batteries. Ethylene carbonate showed good cycling
characteristics [104, 105].
The addition of 2-methylfuran, thiophene, 2-methylthiophene, pyrrole, and
4-methylthiazole to PC-LiPF 6 or PC-THF-LiPF 6 improved the cycling efficiency