Due to the high cost of cobalt, battery manufacturers are switching from cobalt-based cathodes to nickel. Nickel-based systems have higher energy density, lower cost and longer cycle life than cobalt-based batteries, but their voltages are slightly lower. New electrolytes and additives can charge a single battery to more than 4.4V, thereby increasing power. Figure 5 shows the characteristics of NMC.

Because the system's economy and overall performance are relatively good, NMC hybrid lithium-ion batteries are receiving more and more attention. The three active materials, nickel, manganese and cobalt, can be easily blended to suit a wide range of applications in automobiles and energy storage systems that require frequent cycling. The diversity of the NMC family is growing.

Table 6 Characteristics of lithium nickel manganese cobalt oxide (NMC)

4. Lithium Iron Phosphate (LiFePO4)

In 1996, the University of Texas discovered that phosphate could be used as a cathode material for rechargeable lithium batteries. Lithium phosphate has good electrochemical properties and low resistance. This is achieved through nanoscale phosphate cathode materials. The main advantages are high current rating and long cycle life; good thermal stability, enhanced safety and tolerance to abuse.

Lithium phosphate is more tolerant to the full range of charging conditions and is less stressed than other lithium-ion systems if left at high voltage for extended periods of time. The disadvantage is that the lower nominal battery voltage of 3.2V results in lower specific energy than cobalt-doped lithium-ion batteries. For most batteries, low temperatures will reduce performance and elevated storage temperatures will shorten service life, and lithium phosphate is no exception. Lithium phosphate has a higher self-discharge than other lithium-ion batteries, which may cause aging and thus equalization issues. Although this can be compensated for by selecting a high-quality battery or using an advanced battery management system, both methods increase the the cost of the battery pack. Battery life is very sensitive to impurities in the manufacturing process and cannot withstand moisture doping. Due to the presence of moisture impurities, some batteries have a shortest life of only 50 cycles.

Figure 7 Spider diagram of a typical lithium phosphate battery

Lithium phosphate is commonly used instead of lead-acid starter batteries. Four batteries in series produce 12.80V, which is similar to six 2V lead-acid batteries in series. The vehicle charges the lead acid to 14.40V (2.40V/battery) and maintains the float charge state. The purpose of float charging is to maintain a full charge level and prevent sulfation of lead-acid batteries.

By connecting four lithium phosphate batteries in series, the voltage of each battery is 3.60V, which is the correct full charge voltage. At this point, charging should be disconnected, but charging should continue while driving. Lithium phosphate tolerates some overcharging; however, since most vehicles maintain the voltage at 14.40V for extended periods of time during long journeys, this may increase the mechanical stress on the lithium phosphate battery. Time will tell how long lithium phosphate can withstand overcharging as a replacement for lead-acid batteries. Low temperatures also reduce the performance of lithium ions, potentially affecting cranking capabilities in extreme situations.

5. Lithium Nickel Cobalt Aluminate (NCA)

Lithium nickel cobalt aluminate batteries, or NCA, have been used since 1999. It has high specific energy, quite good specific power and long service life, which are similar to NMC. Less flattering are safety and cost.

Figure 9 Spider diagram of NCA

NCA is a further development of lithium nickel oxide; the addition of aluminum gives the battery better chemical stability. High energy and power density and good service life make NCA a candidate for EV powertrains. High cost and marginal safety have negative consequences.

6. Lithium titanate

Batteries with lithium titanate anodes have been known since the 1980s. Lithium titanate replaces graphite in typical lithium-ion battery negative electrodes, and the material forms a spinel structure. The positive electrode can be lithium manganate or NMC. Lithium titanate has a nominal battery voltage of 2.40V, can be charged quickly, and provides a high discharge current of 10C. The cycle number is said to be higher than that of conventional lithium-ion batteries. Lithium titanate is safe and has excellent low-temperature discharge characteristics, achieving 80% capacity at -30°C (-22°F).

Figure 11 Lithium titanate spider diagram

LTO (usually Li4Ti5O12) has zero strain, no SEI film formation and no lithium plating phenomenon during fast charging and low temperature charging, so it has better charge and discharge performance than traditional cobalt-doped Li-ions and graphite anodes. Thermal stability at high temperatures is also better than other lithium-ion systems; however, the batteries are expensive. The specific energy is low, only 65Wh/kg, which is equivalent to NiCd. Lithium titanate charges to 2.80V and ends at 1.80V at the end of discharge. Figure 13 shows the characteristics of lithium titanate batteries. Typical uses are electric powertrains, UPS and solar street lights.