110
down at higher voltages, and therefore this limitation inhibits the energy storage
capacity for aqueous batteries. Great interest exists in secondary batteries with high
voltages to give lightweight devices, driven by advancements in cell phones, laptops,
smartwatches, etc. Hence, the non-aqueous electrolytes with higher voltage stability
that are necessary for lithium ion batteries are the focus of intense research efforts.
5.1 Lithium Ion Batteries
Lithium is an ideal anode material for high energy batteries for two major reasons:
it is the lightest elemental metal and it possesses a very large negative standard
reduction potential (−3.0 V vs. NHE). This affords lithium batteries a superior
energy storage capability compared to aqueous systems. Organic liquid electrolytes
(e.g. ethers, carbonates) are often used as they offer stability at the operating potentials; however, they present a safety risk due to their potential flammability after a
catastrophic event (e.g. a short circuit whereby significant energy could be released
in a very short time period). Although invaluable for high energy primary batteries,
lithium metal anodes do not recharge effectively, forming high surface area dendritic structures upon repeated cycling that are reactive with the electrolyte and that
can, potentially, short the battery. However, the discovery of highly reversible Li
+
intercalation into suitable anode (e.g. carbon) and cathode (e.g. metal oxides) materials has allowed for the development of Li-ion rechargeable chemistries that operate with similar energies to the primary batteries but avoid the formation of Li metal
in the electrochemical reactions (see Fig. 6, top for a simplified schematic).
Lithium ion batteries have been a boon for portable electronics. The use of ion
intercalation electrodes is one of the most important advances made in the electronics industry, and the first major change in battery technology from the basic Galvanic
cell model of metal ions reducing at the cathode and oxidizing at the anode. In most
commercial devices, the electrolyte consists of lithium salts with a suitable anion to
allow for solubility in the non-aqueous solvents used (e.g. LiPF 6 or LiBF 4 in ethylene carbonate/diethyl carbonate). Conductivities are relatively high in these liquid
systems, approximately 10
−2
S cm
−1
at room temperature. However, the liquid electrolyte still presents a flammability risk so the search for solid-state electrolytes that
are compatible with lithium ion batteries is well underway in an attempt to prevent
leakage, flammability, and solvent breakdown at the electrodes. A typical figure-ofmerit for a good solid-state Li
+
conductor is one with conductivities that are above
10
−4
S cm
−1
at room temperature [6].
Although lithium ions offer higher energy densities than other alkali metal ions
due to their favorable mass/voltage, sodium and potassium ion batteries are considered to be attractive alternatives due to the high abundance of these elements.
Additionally, magnesium ion batteries are considered to be the “holy grail” for
some [91], but there are very few options for liquid electrolytes and magnesium ions
are thought to move too slowly in the solid state. Relatively minimal, but notable,
work has been conducted on these non-lithium-containing systems and will be
briefly discussed here.
C. A. Bauer
down at higher voltages, and therefore this limitation inhibits the energy storage
capacity for aqueous batteries. Great interest exists in secondary batteries with high
voltages to give lightweight devices, driven by advancements in cell phones, laptops,
smartwatches, etc. Hence, the non-aqueous electrolytes with higher voltage stability
that are necessary for lithium ion batteries are the focus of intense research efforts.
5.1 Lithium Ion Batteries
Lithium is an ideal anode material for high energy batteries for two major reasons:
it is the lightest elemental metal and it possesses a very large negative standard
reduction potential (−3.0 V vs. NHE). This affords lithium batteries a superior
energy storage capability compared to aqueous systems. Organic liquid electrolytes
(e.g. ethers, carbonates) are often used as they offer stability at the operating potentials; however, they present a safety risk due to their potential flammability after a
catastrophic event (e.g. a short circuit whereby significant energy could be released
in a very short time period). Although invaluable for high energy primary batteries,
lithium metal anodes do not recharge effectively, forming high surface area dendritic structures upon repeated cycling that are reactive with the electrolyte and that
can, potentially, short the battery. However, the discovery of highly reversible Li
+
intercalation into suitable anode (e.g. carbon) and cathode (e.g. metal oxides) materials has allowed for the development of Li-ion rechargeable chemistries that operate with similar energies to the primary batteries but avoid the formation of Li metal
in the electrochemical reactions (see Fig. 6, top for a simplified schematic).
Lithium ion batteries have been a boon for portable electronics. The use of ion
intercalation electrodes is one of the most important advances made in the electronics industry, and the first major change in battery technology from the basic Galvanic
cell model of metal ions reducing at the cathode and oxidizing at the anode. In most
commercial devices, the electrolyte consists of lithium salts with a suitable anion to
allow for solubility in the non-aqueous solvents used (e.g. LiPF 6 or LiBF 4 in ethylene carbonate/diethyl carbonate). Conductivities are relatively high in these liquid
systems, approximately 10
−2
S cm
−1
at room temperature. However, the liquid electrolyte still presents a flammability risk so the search for solid-state electrolytes that
are compatible with lithium ion batteries is well underway in an attempt to prevent
leakage, flammability, and solvent breakdown at the electrodes. A typical figure-ofmerit for a good solid-state Li
+
conductor is one with conductivities that are above
10
−4
S cm
−1
at room temperature [6].
Although lithium ions offer higher energy densities than other alkali metal ions
due to their favorable mass/voltage, sodium and potassium ion batteries are considered to be attractive alternatives due to the high abundance of these elements.
Additionally, magnesium ion batteries are considered to be the “holy grail” for
some [91], but there are very few options for liquid electrolytes and magnesium ions
are thought to move too slowly in the solid state. Relatively minimal, but notable,
work has been conducted on these non-lithium-containing systems and will be
briefly discussed here.
C. A. Bauer