This is because the Li+ ion has a very high hydration energy in the gas phase: though the lithium ion disrupts the structure of water significantly, causing a higher change in entropy, this high hydration energy is enough to make the reduction potentials indicate it as being the most electropositive alkali metal, despite the difficulty of ionising it in the gas phase.
ðɪs ɪz bɪkˈʌz ðə lˈaɪ plˈʌs ˈaɪɑːn hɐz ɐ vˈɛɹi hˈaɪ haɪdɹˈeɪʃən ˈɛnɚdʒi ɪnðə ɡˈæs fˈeɪz: ðˌoʊ ðə lˈɪθiəm ˈaɪɑːn dɪsɹˈʌpts ðə stɹˈʌktʃɚɹ ʌv wˈɔːɾɚ sɪɡnˈɪfɪkəntli, kˈɔːzɪŋ ɐ hˈaɪɚ tʃˈeɪndʒ ɪn ˈɛntɹəpi, ðɪs hˈaɪ haɪdɹˈeɪʃən ˈɛnɚdʒi ɪz ɪnˈʌf tə mˌeɪk ðə ɹᵻdˈʌkʃən pətˈɛnʃəlz ˈɪndᵻkˌeɪt ɪɾ æz bˌiːɪŋ ðə mˈoʊst ɪlˌɛktɹoʊpˈɑːzᵻtˌɪv ˈælkəlˌaɪ mˈɛɾəl, dᵻspˈaɪt ðə dˈɪfɪkˌʌlti ʌv ˈaɪənˌaɪzɪŋ ɪɾ ɪnðə ɡˈæs fˈeɪz.